Perl Interview Questions

  1. What arguments do you frequently use for the Perl interpreter and what do they mean?
  2. What does the command ‘use strict’ do and why should you use it?
  3. What do the symbols $ @ and % mean when prefixing a variable?
  4. What elements of the Perl language could you use to structure your code to allow for maximum re-use and maximum readability?
  5. What are the characteristics of a project that is well suited to Perl?

  6. Why do you program in Perl?
  7. Explain the difference between my and local.
  8. Explain the difference between use and require.
  9. What’s your favorite module and why?
  10. What is a hash?
  11. Write a simple (common) regular expression to match an IP address, e-mail address, city-state-zipcode combination.
  12. What purpose does each of the following serve: -w, strict, -T ?
  13. What is the difference between for & foreach, exec & system?
  14. Where do you go for Perl help?
  15. Name an instance where you used a CPAN module.
  16. How do you open a file for writing?
  17. How would you replace a char in string and how do you store the number of replacements?
  18. When would you not use Perl for a project?
  19. What is the simplest type of data that Perl can work with?
  20. Which operator can be used to take the bottom item from an array?
  21. You want to close the directory handle EVAN. Which of the following should you use to accomplish this?
  22. You want your script to read in a value from the keyboard for a variable named $duo. What do you set $duo equal to in the program to have it read a keyboard value? __________ (Fill in the blank.)
  23. Your script has just read in a variable from the keyboard and you want to strip that variable of the newline character that came in. What operator should you use to perform this operation?
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Microsoft SQL Interview Questions

  1. Which of the following has the highest order of precedence?
    • Functions and Parenthesis
    • Multiplication, Division and Exponents
    • Addition and Subtraction
    • Logical Operations
  2. When designing a database table, how do you avoid missing column values for non-primary key columns?
    • Use UNIQUE constraints
    • Use PRIMARY KEY constraints
    • Use DEFAULT and NOT NULL constraints
    • Use FOREIGN KEY constraints
    • Use SET constraints

  3. Which of the following is the syntax for creating an Index?
    • CREATE [UNIQUE] INDEX index_name OF tbl_name (index_columns)
    • CREATE [UNIQUE] INDEX OF tbl_name (index_columns)
    • CREATE [UNIQUE] INDEX ON tbl_name (index_columns)
    • CREATE [UNIQUE] INDEX index_name ON tbl_name (index_columns)
  4. Which of the following is not a valid character datatype in SQL Server?
    • BLOB
    • CHAR
    • VARCHAR
    • TEXT
    • VARTEXT
  5. Which of the following statements about SQL Server comments is false?
    • /* … */ are used for multiline comments
    • // is used for single line comments
    • – is used for single line comments
    • Nested comments are allowed i.e. /* comment 1 /* comment 2 */ comment 1*/
    • ‘ is used for single line comments
  6. Consider the following transaction code:

    Begin Transaction
    Update names_table set employee_name = "Ramesh" where employee_name = "Mahesh"
    Save Transaction SAVE_POINT
    Update salaries set salary=salary + 900 where employee_job = "Engineer"
    Rollback transaction
    Commit transaction
    What will be the result produced by this transaction?
    • “Ramesh” will be updated to “Mahesh”, but salaries of engineers will not be

      • updated

    • Neither “Ramesh” will be updated to “Mahesh”, nor the salary of engineers will be updated.
    • “Ramesh” will be updated to “Mahesh” and salary of engineers will also be

      • updated.

  7. Which of the following constraints can be used to enforce the uniqueness of rows in a table?
    • DEFAULT and NOT NULL constraints
    • FOREIGN KEY constraints
    • PRIMARY KEY and UNIQUE constraints
    • IDENTITY columns
    • CHECK constraints
  8. Which of the following are not date parts?
    • quarter
    • dayofweek
    • dayofyear
    • weekday
  9. The IF UPDATE (column_name) parameter in a trigger definition will return
    TRUE in case of an INSERT statement being executed on the triggered table:
    • Yes
    • No
    • It returns TRUE only if an UPDATE query is executed
    • Both b and c
  10. Which one of the following must be specified in every DELETE statement?
    • Table Name
    • Database name
    • LIMIT clause
    • WHERE clause
    • Column Names
  11. Which one of the following correctly selects rows from the table myTable that have null in column column1?
    • SELECT * FROM myTable WHERE column1 is null
    • SELECT * FROM myTable WHERE column1 = null
    • SELECT * FROM myTable WHERE column1 EQUALS null
    • SELECT * FROM myTable WHERE column1 NOT null
    • SELECT * FROM myTable WHERE column1 CONTAINS null
  12. Is this statement true or false:
    A cursor is a pointer that identifies a specific working row within a set
    • True
    • False
  13. Which of the following commands is used to change the structure of table?
    • CHANGE TABLE
    • MODIFY TABLE
    • ALTER TABLE
    • UPDATE TABLE
  14. Consider the following statements and pick the correct answer:

    1. ceiling() - returns the smallest integer greater than or equal to the specified value
    2. floor() - returns the largest integer less than or equal to the specified value
    • 1 is true and 2 is false
    • 1 is false and 2 is true
    • Both 1 and 2 are true
    • Both 1 and 2 are false
  15. What is the correct SQL syntax for returning all the columns from a table named “Persons” sorted REVERSE alphabetically by “FirstName”?
    • SELECT * FROM Persons WHERE FirstName ORDER BY FirstName DESC
    • SELECT * FROM Persons SORT REVERSE ‘FirstName’

      • c . SELECT * FROM Persons ORDER BY ‘FirstName’

    • SELECT * FROM Persons ORDER BY FirstName DESC

      • SELECT * FROM Persons ORDER BY DESC FirstName

  16. What is the maximum value that can be stored for a datetime field?
    • Dec 31, 9999
    • Jun 6, 2079
    • Jan 1, 2753
    • Jan 1, 2100
  17. Consider the following queries:
    1. select * from employee where department LIKE "[^F-M]%”;
    2. select * from employee where department = “[^F-M]%”;
    Select the correct option:
    • Query 2 will return an error
    • Both the queries will return the same set of records
    • Query 2 is perfectly correct
    • Query 2 would return one record less than Query 1
  18. How can you view the structure of a table named “myTable” in SQL Server?
    • desc myTable
    • desc table myTable
    • sp_columns myTable
    • None of the above
    • Using either option a or c
  19. What does referential integrity (also called relational integrity) prevent?
    • Loss of data from employee sabotage
    • Loss of data from any one corrupted table
    • Recursive joins
    • One-to-many or many-to-many relationships between columns in a table
    • Data redundancy
  20. Which of the following is not a global variable?
    • @@colcount
    • @@error
    • @@rowcount
    • @@version
    • All are valid global variables
  21. Consider the following two tables:

    1. customers( customer_id, customer_name)
    2. branch ( branch_id, branch_name )
    What will be the output if the following query is executed:
    Select * branch_name from customers,branch
    • It will return the fields customer_id, customer_name, branch_name
    • It will return the fields customer_id, customer_name, branch_id, branch_name
    • It will return the fields customer_id, customer_name, branch_id, branch_name, branch_name
    • It will return an empty set since the two tables do not have any common field name
    • It will return an error since * is used alone for one table only
  22. Which of the following is not a control statement?
    • if…else
    • if exists
    • do…while
    • while
    • begin…end
  23. Which of the following is not a valid Numeric datatypes in SQL Server?
    • INT
    • SMALLINT
    • TINYINT
    • BIGINT
    • MONEY
  24. Which of the following datatypes is not supported by SQL-Server?
    • Character
    • Binary
    • Logical
    • Date
    • Numeric
    • All are supported
  25. What will the output be if you try to perform arithmetic on NULL values?
    • 0
    • NULL
    • It will generate an error message
    • Can’t be determined
  26. Which of the following options is not correct about the DATEDIFF() function?
    • It returns the difference between parts of two specified dates
    • It takes three arguments
    • It returns a signed integer value equal to second date part minus first date part
    • It returns a signed integer value equal to first date part minus second date part
  27. Sample Code

    CREATE TABLE table1(
    column1 varchar(50),
    column2 varchar(50),
    column3 varchar(50),
    column4 varchar(50));
    Which one of the following is the correct syntax for adding the column named “column2a” after column2 to the table shown above?
    • ALTER TABLE table1 ADD column2a varchar(50) AFTER column2;
    • MODIFY TABLE table1 ADD column2a AFTER column2;
    • INSERT INTO table1 column2a AS varchar(50) AFTER column2;
    • ALTER TABLE table1 INSERT column2a varchar(50) AFTER column2;
    • CHANGE TABLE table1 INSERT column2a BEFORE column3;
  28. State which of the following are true
    • Views are a logical way of looking at the logical data located in the tables
    • Views are a logical way of looking at the physical data located in the tables
    • Tables are physical constructs used for storage and manipulation of data in databases
    • Tables are logical constructs used for storage and manipulation of data in databases
  29. Which of the following is not a valid binary datatype in SQL Server?
    • BINARY
    • VARBINARY
    • BIT
    • IMAGE
    • TESTAMP
  30. Which of the following is false with regards to sp_help?
    • When a procedure name is passed to sp_help, it shows the parameters
    • When a table name is passed to sp_help, it shows the structure of the table
    • When no parameter is passed, it provides a list of all objects and user-defined datatypes in a database
    • All of the above are true
    • Which of the following are false for batches (batch commands)?
      • Statements in a batch are parsed, compiled and executed as a group
      • None of the statements in the batch is executed if there are any syntax errors in the batch
      • None of the statements in the batch is executed if there are any parsing errors in the batch
      • None of the statements in the batch is executed if there are any fatal errors in the batch
    • Select the correct option:
      • Optimistic locking is a locking scheme handled by the server, whereas pessimistic locking is handled by the application developer
      • Pessimistic locking is a locking scheme handled by the server, whereas optimistic locking is handled by the application developer
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Linux MySQL Interview Questions

  1. How do you start and stop MySQL on Windows? - net start MySQL, net stop MySQL
  2. How do you start MySQL on Linux? - /etc/init.d/mysql start
  3. Explain the difference between mysql and mysqli interfaces in PHP? - mysqli is the object-oriented version of mysql library functions.
  4. What’s the default port for MySQL Server? - 3306
  5. What does tee command do in MySQL? - tee followed by a filename turns on MySQL logging to a specified file. It can be stopped by command notee.
  6. Can you save your connection settings to a conf file? - Yes, and name it ~/.my.conf. You might want to change the permissions on the file to 600, so that it’s not readable by others.
  7. How do you change a password for an existing user via mysqladmin? - mysqladmin -u root -p password "newpassword"
  8. Use mysqldump to create a copy of the database? - mysqldump -h mysqlhost -u username -p mydatabasename > dbdump.sql
  9. Have you ever used MySQL Administrator and MySQL Query Browser? Describe the tasks you accomplished with these tools.
  10. What are some good ideas regarding user security in MySQL? - There is no user without a password. There is no user without a user name. There is no user whose Host column contains % (which here indicates that the user can log in from anywhere in the network or the Internet). There are as few users as possible (in the ideal case only root) who have unrestricted access.
  11. Explain the difference between MyISAM Static and MyISAM Dynamic. - In MyISAM static all the fields have fixed width. The Dynamic MyISAM table would include fields such as TEXT, BLOB, etc. to accommodate the data types with various lengths. MyISAM Static would be easier to restore in case of corruption, since even though you might lose some data, you know exactly where to look for the beginning of the next record.
  12. What does myisamchk do? - It compressed the MyISAM tables, which reduces their disk usage.
  13. Explain advantages of InnoDB over MyISAM? - Row-level locking, transactions, foreign key constraints and crash recovery.
  14. Explain advantages of MyISAM over InnoDB? - Much more conservative approach to disk space management - each MyISAM table is stored in a separate file, which could be compressed then with myisamchk if needed. With InnoDB the tables are stored in tablespace, and not much further optimization is possible. All data except for TEXT and BLOB can occupy 8,000 bytes at most. No full text indexing is available for InnoDB. TRhe COUNT(*)s execute slower than in MyISAM due to tablespace complexity.
  15. What are HEAP tables in MySQL? - HEAP tables are in-memory. They are usually used for high-speed temporary storage. No TEXT or BLOB fields are allowed within HEAP tables. You can only use the comparison operators = and <=>. HEAP tables do not support AUTO_INCREMENT. Indexes must be NOT NULL.
  16. How do you control the max size of a HEAP table? - MySQL config variable max_heap_table_size.
  17. What are CSV tables? - Those are the special tables, data for which is saved into comma-separated values files. They cannot be indexed.
  18. Explain federated tables. - Introduced in MySQL 5.0, federated tables allow access to the tables located on other databases on other servers.
  19. What is SERIAL data type in MySQL? - BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT
  20. What happens when the column is set to AUTO INCREMENT and you reach the maximum value for that table? - It stops incrementing. It does not overflow to 0 to prevent data losses, but further inserts are going to produce an error, since the key has been used already.
  21. Explain the difference between BOOL, TINYINT and BIT. - Prior to MySQL 5.0.3: those are all synonyms. After MySQL 5.0.3: BIT data type can store 8 bytes of data and should be used for binary data.
  22. Explain the difference between FLOAT, DOUBLE and REAL. - FLOATs store floating point numbers with 8 place accuracy and take up 4 bytes. DOUBLEs store floating point numbers with 16 place accuracy and take up 8 bytes. REAL is a synonym of FLOAT for now.
  23. If you specify the data type as DECIMAL (5,2), what’s the range of values that can go in this table? - 999.99 to -99.99. Note that with the negative number the minus sign is considered one of the digits.
  24. What happens if a table has one column defined as TIMESTAMP? - That field gets the current timestamp whenever the row gets altered.
  25. But what if you really want to store the timestamp data, such as the publication date of the article? - Create two columns of type TIMESTAMP and use the second one for your real data.
  26. Explain data type TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP - The column exhibits the same behavior as a single timestamp column in a table with no other timestamp columns.
  27. What does TIMESTAMP ON UPDATE CURRENT_TIMESTAMP data type do? - On initialization places a zero in that column, on future updates puts the current value of the timestamp in.
  28. Explain TIMESTAMP DEFAULT ‘2006:09:02 17:38:44′ ON UPDATE CURRENT_TIMESTAMP. - A default value is used on initialization, a current timestamp is inserted on update of the row.
  29. If I created a column with data type VARCHAR(3), what would I expect to see in MySQL table? - CHAR(3), since MySQL automatically adjusted the data type.
  30. Explain MySQL architecture. - The front layer takes care of network connections and security authentications, the middle layer does the SQL query parsing, and then the query is handled off to the storage engine. A storage engine could be either a default one supplied with MySQL (MyISAM) or a commercial one supplied by a third-party vendor (ScaleDB, InnoDB, etc.)
  31. Explain MySQL locks. - Table-level locks allow the user to lock the entire table, page-level locks allow locking of certain portions of the tables (those portions are referred to as tables), row-level locks are the most granular and allow locking of specific rows.
  32. Explain multi-version concurrency control in MySQL. - Each row has two additional columns associated with it - creation time and deletion time, but instead of storing timestamps, MySQL stores version numbers.
  33. What are MySQL transactions? - A set of instructions/queries that should be executed or rolled back as a single atomic unit.
  34. What’s ACID? - Automicity - transactions are atomic and should be treated as one in case of rollback. Consistency - the database should be in consistent state between multiple states in transaction. Isolation - no other queries can access the data modified by a running transaction. Durability - system crashes should not lose the data.
  35. Which storage engines support transactions in MySQL? - Berkeley DB and InnoDB.
  36. How do you convert to a different table type? - ALTER TABLE customers TYPE = InnoDB
  37. How do you index just the first four bytes of the column? - ALTER TABLE customers ADD INDEX (business_name(4))
  38. What’s the difference between PRIMARY KEY and UNIQUE in MyISAM? - PRIMARY KEY cannot be null, so essentially PRIMARY KEY is equivalent to UNIQUE NOT NULL.
  39. How do you prevent MySQL from caching a query? - SELECT SQL_NO_CACHE …
  40. What’s the difference between query_cache_type 1 and 2? - The second one is on-demand and can be retrieved via SELECT SQL_CACHE … If you’re worried about the SQL portability to other servers, you can use SELECT /* SQL_CACHE */ id FROM … - MySQL will interpret the code inside comments, while other servers will ignore it.
  41. What is DDL, DML and DCL? - If you look at the large variety of SQL commands, they can be divided into three large subgroups. Data Definition Language deals with database schemas and descriptions of how the data should reside in the database, therefore language statements like CREATE TABLE or ALTER TABLE belong to DDL. DML deals with data manipulation, and therefore includes most common SQL statements such SELECT, INSERT, etc. Data Control Language includes commands such as GRANT, and mostly concerns with rights, permissions and other controls of the database system.
  42. How do you get the number of rows affected by query? - SELECT COUNT (user_id) FROM users would only return the number of user_id’s.
  43. If the value in the column is repeatable, how do you find out the unique values? - Use DISTINCT in the query, such as SELECT DISTINCT user_firstname FROM users; You can also ask for a number of distinct values by saying SELECT COUNT (DISTINCT user_firstname) FROM users;
  44. How do you return the a hundred books starting from 25th? - SELECT book_title FROM books LIMIT 25, 100. The first number in LIMIT is the offset, the second is the number.
  45. You wrote a search engine that should retrieve 10 results at a time, but at the same time you’d like to know how many rows there’re total. How do you display that to the user? - SELECT SQL_CALC_FOUND_ROWS page_title FROM web_pages LIMIT 1,10; SELECT FOUND_ROWS(); The second query (not that COUNT() is never used) will tell you how many results there’re total, so you can display a phrase "Found 13,450,600 results, displaying 1-10". Note that FOUND_ROWS does not pay attention to the LIMITs you specified and always returns the total number of rows affected by query.
  46. How would you write a query to select all teams that won either 2, 4, 6 or 8 games? - SELECT team_name FROM teams WHERE team_won IN (2, 4, 6, 8)
  47. How would you select all the users, whose phone number is null? - SELECT user_name FROM users WHERE ISNULL(user_phonenumber);
  48. What does this query mean: SELECT user_name, user_isp FROM users LEFT JOIN isps USING (user_id) - It’s equivalent to saying SELECT user_name, user_isp FROM users LEFT JOIN isps WHERE users.user_id=isps.user_id
  49. How do you find out which auto increment was assigned on the last insert? - SELECT LAST_INSERT_ID() will return the last value assigned by the auto_increment function. Note that you don’t have to specify the table name.
  50. What does –i-am-a-dummy flag to do when starting MySQL? - Makes the MySQL engine refuse UPDATE and DELETE commands where the WHERE clause is not present.
  51. On executing the DELETE statement I keep getting the error about foreign key constraint failing. What do I do? - What it means is that so of the data that you’re trying to delete is still alive in another table. Like if you have a table for universities and a table for students, which contains the ID of the university they go to, running a delete on a university table will fail if the students table still contains people enrolled at that university. Proper way to do it would be to delete the offending data first, and then delete the university in question. Quick way would involve running SET foreign_key_checks=0 before the DELETE command, and setting the parameter back to 1 after the DELETE is done. If your foreign key was formulated with ON DELETE CASCADE, the data in dependent tables will be removed automatically.
  52. When would you use ORDER BY in DELETE statement? - When you’re not deleting by row ID. Such as in DELETE FROM techinterviews_com_questions ORDER BY timestamp LIMIT 1. This will delete the most recently posted question in the table techinterviews_com_questions.
  53. How can you see all indexes defined for a table? - SHOW INDEX FROM techinterviews_questions;
  54. How would you change a column from VARCHAR(10) to VARCHAR(50)? - ALTER TABLE techinterviews_questions CHANGE techinterviews_content techinterviews_CONTENT VARCHAR(50).
  55. How would you delete a column? - ALTER TABLE techinterviews_answers DROP answer_user_id.
  56. How would you change a table to InnoDB? - ALTER TABLE techinterviews_questions ENGINE innodb;
  57. When you create a table, and then run SHOW CREATE TABLE on it, you occasionally get different results than what you typed in. What does MySQL modify in your newly created tables? -
    1. VARCHARs with length less than 4 become CHARs
    2. CHARs with length more than 3 become VARCHARs.
    3. NOT NULL gets added to the columns declared as PRIMARY KEYs
    4. Default values such as NULL are specified for each column
  58. How do I find out all databases starting with ‘tech’ to which I have access to? - SHOW DATABASES LIKE ‘tech%’;
  59. How do you concatenate strings in MySQL? - CONCAT (string1, string2, string3)
  60. How do you get a portion of a string? - SELECT SUBSTR(title, 1, 10) from techinterviews_questions;
  61. What’s the difference between CHAR_LENGTH and LENGTH? - The first is, naturally, the character count. The second is byte count. For the Latin characters the numbers are the same, but they’re not the same for Unicode and other encodings.
  62. How do you convert a string to UTF-8? - SELECT (techinterviews_question USING utf8);
  63. What do % and _ mean inside LIKE statement? - % corresponds to 0 or more characters, _ is exactly one character.
  64. What does + mean in REGEXP? - At least one character. Appendix G. Regular Expressions from MySQL manual is worth perusing before the interview.
  65. How do you get the month from a timestamp? - SELECT MONTH(techinterviews_timestamp) from techinterviews_questions;
  66. How do you offload the time/date handling to MySQL? - SELECT DATE_FORMAT(techinterviews_timestamp, ‘%Y-%m-%d’) from techinterviews_questions; A similar TIME_FORMAT function deals with time.
  67. How do you add three minutes to a date? - ADDDATE(techinterviews_publication_date, INTERVAL 3 MINUTE)
  68. What’s the difference between Unix timestamps and MySQL timestamps? - Internally Unix timestamps are stored as 32-bit integers, while MySQL timestamps are stored in a similar manner, but represented in readable YYYY-MM-DD HH:MM:SS format.
  69. How do you convert between Unix timestamps and MySQL timestamps? - UNIX_TIMESTAMP converts from MySQL timestamp to Unix timestamp, FROM_UNIXTIME converts from Unix timestamp to MySQL timestamp.
  70. What are ENUMs used for in MySQL? - You can limit the possible values that go into the table. CREATE TABLE months (month ENUM ‘January’, ‘February’, ‘March’,…); INSERT months VALUES (’April’);
  71. How are ENUMs and SETs represented internally? - As unique integers representing the powers of two, due to storage optimizations.
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SQL Interview Questions

  1. What are two methods of retrieving SQL?
  2. What cursor type do you use to retrieve multiple recordsets?
  3. What is the difference between a "where" clause and a "having" clause? - "Where" is a kind of restiriction statement. You use where clause to restrict all the data from DB.Where clause is using before result retrieving. But Having clause is using after retrieving the data.Having clause is a kind of filtering command.
  4. What is the basic form of a SQL statement to read data out of a table? The basic form to read data out of table is ‘SELECT * FROM table_name; ‘ An answer: ‘SELECT * FROM table_name WHERE xyz= ‘whatever’;’ cannot be called basic form because of WHERE clause.
  5. What structure can you implement for the database to speed up table reads? - Follow the rules of DB tuning we have to: 1] properly use indexes ( different types of indexes) 2] properly locate different DB objects across different tablespaces, files and so on.3] create a special space (tablespace) to locate some of the data with special datatype ( for example CLOB, LOB and …)
  6. What are the tradeoffs with having indexes? - 1. Faster selects, slower updates. 2. Extra storage space to store indexes. Updates are slower because in addition to updating the table you have to update the index.
  7. What is a "join"? - ‘join’ used to connect two or more tables logically with or without common field.
  8. What is "normalization"? "Denormalization"? Why do you sometimes want to denormalize? - Normalizing data means eliminating redundant information from a table and organizing the data so that future changes to the table are easier. Denormalization means allowing redundancy in a table. The main benefit of denormalization is improved performance with simplified data retrieval and manipulation. This is done by reduction in the number of joins needed for data processing.
  9. What is a "constraint"? - A constraint allows you to apply simple referential integrity checks to a table. There are four primary types of constraints that are currently supported by SQL Server: PRIMARY/UNIQUE - enforces uniqueness of a particular table column. DEFAULT - specifies a default value for a column in case an insert operation does not provide one. FOREIGN KEY - validates that every value in a column exists in a column of another table. CHECK - checks that every value stored in a column is in some specified list. Each type of constraint performs a specific type of action. Default is not a constraint. NOT NULL is one more constraint which does not allow values in the specific column to be null. And also it the only constraint which is not a table level constraint.
  10. What types of index data structures can you have? - An index helps to faster search values in tables. The three most commonly used index-types are: - B-Tree: builds a tree of possible values with a list of row IDs that have the leaf value. Needs a lot of space and is the default index type for most databases. - Bitmap: string of bits for each possible value of the column. Each bit string has one bit for each row. Needs only few space and is very fast.(however, domain of value cannot be large, e.g. SEX(m,f); degree(BS,MS,PHD) - Hash: A hashing algorithm is used to assign a set of characters to represent a text string such as a composite of keys or partial keys, and compresses the underlying data. Takes longer to build and is supported by relatively few databases.
  11. What is a "primary key"? - A PRIMARY INDEX or PRIMARY KEY is something which comes mainly from
    database theory. From its behavior is almost the same as an UNIQUE INDEX, i.e. there may only be one of each value in this column. If you call such an INDEX PRIMARY instead of UNIQUE, you say something about
    your table design, which I am not able to explain in few words. Primary Key is a type of a constraint enforcing uniqueness and data integrity for each row of a table. All columns participating in a primary key constraint must possess the NOT NULL property.
  12. What is a "functional dependency"? How does it relate to database table design? - Functional dependency relates to how one object depends upon the other in the database. for example, procedure/function sp2 may be called by procedure sp1. Then we say that sp1 has functional dependency on sp2.
  13. What is a "trigger"? - Triggers are stored procedures created in order to enforce integrity rules in a database. A trigger is executed every time a data-modification operation occurs (i.e., insert, update or delete). Triggers are executed automatically on occurance of one of the data-modification operations. A trigger is a database object directly associated with a particular table. It fires whenever a specific statement/type of statement is issued against that table. The types of statements are insert,update,delete and query statements. Basically, trigger is a set of SQL statements A trigger is a solution to the restrictions of a constraint. For instance: 1.A database column cannot carry PSEUDO columns as criteria where a trigger can. 2. A database constraint cannot refer old and new values for a row where a trigger can.
  14. Why can a "group by" or "order by" clause be expensive to process? - Processing of "group by" or "order by" clause often requires creation of Temporary tables to process the results of the query. Which depending of the result set can be very expensive.
  15. What is "index covering" of a query? - Index covering means that "Data can be found only using indexes, without touching the tables"
  16. What types of join algorithms can you have?
  17. What is a SQL view? - An output of a query can be stored as a view. View acts like small table which meets our criterion. View is a precomplied SQL query which is used to select data from one or more tables. A view is like a table but it doesn’t physically take any space. View is a good way to present data in a particular format if you use that query quite often. View can also be used to restrict users from accessing the tables directly.
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Java Interview Questions

Describe what happens when an object is created in Java?

Several things happen in a particular order to ensure the object is constructed properly:
1. Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implementation-specific data includes pointers to class and method data.
2. The instance variables of the objects are initialized to their default values.
3. The constructor for the most derived class is invoked. The first thing a constructor does is call the constructor for its uppercase. This process continues until the constructor for java.lang.Object is called, as java.lang.Object is the base class for all objects in java.
4. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last.

In Java, you can create a String object as below : String str = "abc"; & String str = new String("abc"); Why cant a button object be created as : Button bt = "abc"? Why is it compulsory to create a button object as: Button bt = new Button("abc"); Why this is not compulsory in String’s case?

Button bt1= "abc"; It is because "abc" is a literal string (something slightly different than a String object, by-the-way) and bt1 is a Button object. That simple. The only object in Java that can be assigned a literal String is java.lang.String. Important to not that you are NOT calling a java.lang.String constuctor when you type String s = "abc"; For example String x = "abc"; String y = "abc"; refer to the same object. While String x1 = new String("abc");
String x2 = new String("abc"); refer to two different objects.

What is the advantage of OOP?

You will get varying answers to this question depending on whom you ask. Major advantages of OOP are:
1. Simplicity: software objects model real world objects, so the complexity is reduced and the program structure is very clear;
2. Modularity: each object forms a separate entity whose internal workings are decoupled from other parts of the system;
3. Modifiability: it is easy to make minor changes in the data representation or the procedures in an OO program. Changes inside a class do not affect any other part of a program, since the only public interface that the external world has to a class is through the use of methods;
4. Extensibility: adding new features or responding to changing operating environments can be solved by introducing a few new objects and modifying some existing ones;
5. Maintainability: objects can be maintained separately, making locating and fixing problems easier;
6. Re-usability: objects can be reused in different programs

What are the main differences between Java and C++?

Everything is an object in Java( Single root hierarchy as everything gets derived from java.lang.Object). Java does not have all the complicated aspects of C++ ( For ex: Pointers, templates, unions, operator overloading, structures etc..) The Java language promoters initially said "No pointers!", but when many programmers questioned how you can work without pointers, the promoters began saying "Restricted pointers." You can make up your mind whether it’s really a pointer or not. In any event, there’s no pointer arithmetic. There are no destructors in Java. (automatic garbage collection), Java does not support conditional compile (#ifdef/#ifndef type). Thread support is built into java but not in C++. Java does not support default arguments. There’s no scope resolution operator :: in Java. Java uses the dot for everything, but can get away with it since you can define elements only within a class. Even the method definitions must always occur within a class, so there is no need for scope resolution there either. There’s no "goto " statement in Java. Java doesn’t provide multiple inheritance (MI), at least not in the same sense that C++ does. Exception handling in Java is different because there are no destructors. Java has method overloading, but no operator overloading. The String class does use the + and += operators to concatenate strings and String expressions use automatic type conversion, but that’s a special built-in case. Java is interpreted for the most part and hence platform independent

What are interfaces?

Interfaces provide more sophisticated ways to organize and control the objects in your system.
The interface keyword takes the abstract concept one step further. You could think of it as a “pure” abstract class. It allows the creator to establish the form for a class: method names, argument lists, and return types, but no method bodies. An interface can also contain fields, but The interface keyword takes the abstract concept one step further. You could think of it as a “pure” abstract class. It allows the creator to establish the form for a class: method names, argument lists, and return types, but no method bodies. An interface can also contain fields, but an interface says: “This is what all classes that implement this particular interface will look like.” Thus, any code that uses a particular interface knows what methods might be called for that interface, and that’s all. So the interface is used to establish a “protocol” between classes. (Some object-oriented programming languages have a keyword called protocol to do the same thing.) Typical example from "Thinking in Java":

import java.util.*;
interface Instrument {
int i = 5; // static & final
// Cannot have method definitions:
void play(); // Automatically public
String what();
void adjust();
}
class Wind implements Instrument {
public void play() {
System.out.println("Wind.play()");
public String what() { return "Wind"; }
public void adjust() {}
}

How can you achieve Multiple Inheritance in Java?

Java’s interface mechanism can be used to implement multiple inheritance, with one important difference from c++ way of doing MI: the inherited interfaces must be abstract. This obviates the need to choose between different implementations, as with interfaces there are no implementations.

interface CanFight {
void fight();
interface CanSwim {
void swim();
interface CanFly {
void fly();
class ActionCharacter {
public void fight() {}
class Hero extends ActionCharacter implements CanFight, CanSwim, CanFly {
public void swim() {}
public void fly() {}
}

You can even achieve a form of multiple inheritance where you can use the *functionality* of classes rather than just the interface:

interface A {
void methodA();
}
class AImpl implements A {
void methodA() { //do stuff }
}
interface B {
void methodB();
}
class BImpl implements B {
void methodB() { //do stuff }
}
class Multiple implements A, B {
private A a = new A();
private B b = new B();
void methodA() { a.methodA(); }
void methodB() { b.methodB(); }
}
This completely solves the traditional problems of multiple inheritance in C++ where name clashes occur between multiple base classes. The coder of the derived class will have to explicitly resolve any clashes. Don’t you hate people who point out minor typos? Everything in the previous example is correct, except you need to instantiate an AImpl and BImpl. So class Multiple would look like this:
class Multiple implements A, B {
private A a = new AImpl();
private B b = new BImpl();
void methodA() { a.methodA(); }
void methodB() { b.methodB(); }
}

What is the difference between StringBuffer and String class?

A string buffer implements a mutable sequence of characters. A string buffer is like a String, but can be modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls. The String class represents character strings. All string literals in Java programs, such as "abc" are constant and implemented as instances of this class; their values cannot be changed after they are created. Strings in Java are known to be immutable. What it means is that every time you need to make a change to a String variable, behind the scene, a "new" String is actually being created by the JVM. For an example: if you change your String variable 2 times, then you end up with 3 Strings: one current and 2 that are ready for garbage collection. The garbage collection cycle is quite unpredictable and these additional unwanted Strings will take up memory until that cycle occurs. For better performance, use StringBuffers for string-type data that will be reused or changed frequently. There is more overhead per class than using String, but you will end up with less overall classes and consequently consume less memory. Describe, in general, how java’s garbage collector works? The Java runtime environment deletes objects when it determines that they are no longer being used. This process is known as garbage collection. The Java runtime environment supports a garbage collector that periodically frees the memory used by objects that are no longer needed. The Java garbage collector is a mark-sweep garbage collector that scans Java’s dynamic memory areas for objects, marking those that are referenced. After all possible paths to objects are investigated, those objects that are not marked (i.e. are not referenced) are known to be garbage and are collected. (A more complete description of our garbage collection algorithm might be "A compacting, mark-sweep collector with some conservative scanning".) The garbage collector runs synchronously when the system runs out of memory, or in response to a request from a Java program. Your Java program can ask the garbage collector to run at any time by calling System.gc(). The garbage collector requires about 20 milliseconds to complete its task so, your program should only run the garbage collector when there will be no performance impact and the program anticipates an idle period long enough for the garbage collector to finish its job. Note: Asking the garbage collection to run does not guarantee that your objects will be garbage collected. The Java garbage collector runs asynchronously when the system is idle on systems that allow the Java runtime to note when a thread has begun and to interrupt another thread (such as Windows 95). As soon as another thread becomes active, the garbage collector is asked to get to a consistent state and then terminate.

What’s the difference between == and equals method?

equals checks for the content of the string objects while == checks for the fact that the two String objects point to same memory location ie they are same references.

What are abstract classes, abstract methods?

Simply speaking a class or a method qualified with "abstract" keyword is an abstract class or abstract method. You create an abstract class when you want to manipulate a set of classes through a common interface. All derived-class methods that match the signature of the base-class declaration will be called using the dynamic binding mechanism. If you have an abstract class, objects of that class almost always have no meaning. That is, abstract class is meant to express only the interface and sometimes some default method implementations, and not a particular implementation, so creating an abstract class object makes no sense and are not allowed ( compile will give you an error message if you try to create one). An abstract method is an incomplete method. It has only a declaration and no method body. Here is the syntax for an abstract method declaration: abstract void f(); If a class contains one or more abstract methods, the class must be qualified an abstract. (Otherwise, the compiler gives you an error message.). It’s possible to create a class as abstract without including any abstract methods. This is useful when you’ve got a class in which it doesn’t make sense to have any abstract methods, and yet you want to prevent any instances of that class. Abstract classes and methods are created because they make the abstractness of a class explicit, and tell both the user and the compiler how it was intended to be used.
For example:
abstract class Instrument {
int i; // storage allocated for each
public abstract void play();
public String what() {
return "Instrument";
public abstract void adjust();
}
class Wind extends Instrument {
public void play() {
System.out.println("Wind.play()");
}
public String what() { return "Wind"; }
public void adjust() {}
Abstract classes are classes for which there can be no instances at run time. i.e. the implementation of the abstract classes are not complete. Abstract methods are methods which have no defintion. i.e. abstract methods have to be implemented in one of the sub classes or else that class will also become Abstract.

What is the difference between an Applet and an Application?

A Java application is made up of a main() method declared as public static void that accepts a string array argument, along with any other classes that main() calls. It lives in the environment that the host OS provides. A Java applet is made up of at least one public class that has to be subclassed from java.awt.Applet. The applet is confined to living in the user’s Web browser, and the browser’s security rules, (or Sun’s appletviewer, which has fewer restrictions). The differences between an applet and an application are as follows:
1. Applets can be embedded in HTML pages and downloaded over the Internet whereas Applications have no special support in HTML for embedding or downloading.
2. Applets can only be executed inside a java compatible container, such as a browser or appletviewer whereas Applications are executed at command line by java.exe or jview.exe.
3. Applets execute under strict security limitations that disallow certain operations(sandbox model security) whereas Applications have no inherent security restrictions.
4. Applets don’t have the main() method as in applications. Instead they operate on an entirely different mechanism where they are initialized by init(),started by start(),stopped by stop() or destroyed by destroy().

Java says "write once, run anywhere". What are some ways this isn’t quite true?

As long as all implementaions of java are certified by sun as 100% pure java this promise of "Write once, Run everywhere" will hold true. But as soon as various java core implemenations start digressing from each other, this won’t be true anymore. A recent example of a questionable business tactic is the surreptitious behavior and interface modification of some of Java’s core classes in their own implementation of Java. Programmers who do not recognize these undocumented changes can build their applications expecting them to run anywhere that Java can be found, only to discover that their code works only on Microsoft’s own Virtual Machine, which is only available on Microsoft’s own operating systems.

What is the difference between a Vector and an Array. Discuss the advantages and disadvantages of both?

Vector can contain objects of different types whereas array can contain objects only of a single type.
- Vector can expand at run-time, while array length is fixed.
- Vector methods are synchronized while Array methods are not

What are java beans?

JavaBeans is a portable, platform-independent component model written in the Java programming language, developed in collaboration with industry leaders. It enables developers to write reusable components once and run them anywhere — benefiting from the platform-independent power of Java technology. JavaBeans acts as a Bridge between proprietary component models and provides a seamless and powerful means for developers to build components that run in ActiveX container applications. JavaBeans are usual Java classes which adhere to certain coding conventions:
1. Implements java.io.Serializable interface
2. Provides no argument constructor
3. Provides getter and setter methods for accessing it’s properties

What is RMI?

RMI stands for Remote Method Invocation. Traditional approaches to executing code on other machines across a network have been confusing as well as tedious and error-prone to implement. The nicest way to think about this problem is that some object happens to live on another machine, and that you can send a message to the remote object and get a result as if the object lived on your local machine. This simplification is exactly what Java Remote Method Invocation (RMI) allows you to do. Above excerpt is from "Thinking in java". For more information refer to any book on Java.

What does the keyword "synchronize" mean in java. When do you use it? What are the disadvantages of synchronization?

Synchronize is used when you want to make your methods thread safe. The disadvantage of synchronize is it will end up in slowing down the program. Also if not handled properly it will end up in dead lock.

What gives java it’s "write once and run anywhere" nature?

Java is compiled to be a byte code which is the intermediate language between source code and machine code. This byte code is not platorm specific and hence can be fed to any platform. After being fed to the JVM, which is specific to a particular operating system, the code platform specific machine code is generated thus making java platform independent.

What are native methods? How do you use them?

Native methods are methods written in other languages like C, C++, or even assembly language. You can call native methods from Java using JNI. Native methods are used when the implementation of a particular method is present in language other than Java say C, C++. To use the native methods in java we use the keyword native
public native method_a(). This native keyword is signal to the java compiler that the implementation of this method is in a language other than java. Native methods are used when we realize that it would take up a lot of rework to write that piece of already existing code in other language to Java.

What is JDBC? Describe the steps needed to execute a SQL query using JDBC.

We can connect to databases from java using JDBC. It stands for Java DataBase Connectivity.
Here are the steps:
1. Register the jdbc driver with the driver manager
2. Establish jdbc connection
3. Execute an sql statement
4. Process the results
5. Close the connection
Before doing these do import java.sql.*
JDBC is java based API for accessing data from the relational databases. JDBC provides a set of classes and interfaces for doing various database operations. The steps are:
Register/load the jdbc driver with the driver manager.
Establish the connection thru DriverManager.getConnection();
Fire a SQL thru conn.executeStatement();
Fetch the results in a result set
Process the results
Close statement/result set and connection object.

How many different types of JDBC drivers are present? Discuss them.

There are four JDBC driver types.
Type 1: JDBC-ODBC Bridge plus ODBC Driver:
The first type of JDBC driver is the JDBC-ODBC Bridge. It is a driver that provides JDBC access to databases through ODBC drivers. The ODBC driver must be configured on the client for the bridge to work. This driver type is commonly used for prototyping or when there is no JDBC driver available for a particular DBMS.
Type 2: Native-API partly-Java Driver:
The Native to API driver converts JDBC commands to DBMS-specific native calls. This is much like the restriction of Type 1 drivers. The client must have some binary code loaded on its machine. These drivers do have an advantage over Type 1 drivers because they interface directly with the database.
Type 3: JDBC-Net Pure Java Driver:
The JDBC-Net drivers are a three-tier solution. This type of driver translates JDBC calls into a database-independent network protocol that is sent to a middleware server. This server then translates this DBMS-independent protocol into a DBMS-specific protocol, which is sent
to a particular database. The results are then routed back through the middleware server and sent back to the client. This type of solution makes it possible to implement a pure Java client. It also makes it possible to swap databases without affecting the client.
Type 4: Native-Protocol Pure Java Driver
These are pure Java drivers that communicate directly with the vendor’s database. They do this by converting JDBC commands directly into the database engine’s native protocol. This driver has no additional translation or middleware layer, which improves performance tremendously.
What does the "static" keyword mean in front of a variable? A method? A class? Curly braces {}?
static variable
- means a class level variable
static method:
-does not have "this". It is not allowed to access the not static members of the class.
can be invoked enev before a single instance of a class is created.
eg: main
static class:
no such thing.
static free floating block:
is executed at the time the class is loaded. There can be multiple such blocks. This may be useful to load native libraries when using native methods.
eg:
native void doThis(){
static{
System.loadLibrary("myLibrary.lib");
}

Access specifiers: "public", "protected", "private", nothing?

In the case of Public, Private and Protected, that is used to describe which programs can access that class or method: Public – any other class from any package can instantiate and execute the classes and methods. Protected – only subclasses and classes inside of the package can access the classes and methods. Private – the original class is the only class allowed to executed the methods.

What does the "final" keyword mean in front of a variable? A method? A class?

FINAL for a variable : value is constant
FINAL for a method : cannot be overridden
FINAL for a class : cannot be derived
A final variable cannot be reassigned,
but it is not constant. For instance,
final StringBuffer x = new StringBuffer()
x.append("hello");
is valid. X cannot have a new value in it,but nothing stops operations on the object
that it refers, including destructive operations. Also, a final method cannot be overridden
or hidden by new access specifications.This means that the compiler can choose
to in-line the invocation of such a method.(I don’t know if any compiler actually does
this, but it’s true in theory.) The best example of a final class is
String, which defines a class that cannot be derived.

Does Java have "goto"?

No.

What synchronization constructs does Java provide? How do they work?

The two common features that are used are:
1. Synchronized keyword - Used to synchronize a method or a block of code. When you synchronize a method, you are in effect synchronizing the code within the method using the monitor of the current object for the lock.
The following have the same effect.
synchronized void foo() {
}
and
void foo() {
synchronized(this) {
}
If you synchronize a static method, then you are synchronizing across all objects of the same class, i.e. the monitor you are using for the lock is one per class, not one per object.
2. wait/notify. wait() needs to be called from within a synchronized block. It will first release the lock acquired from the synchronization and then wait for a signal. In Posix C, this part is equivalent to the pthread_cond_wait method, which waits for an OS signal to continue. When somebody calls notify() on the object, this will signal the code which has been waiting, and the code will continue from that point. If there are several sections of code that are in the wait state, you can call notifyAll() which will notify all threads that are waiting on the monitor for the current object. Remember that both wait() and notify() have to be called from blocks of code that are synchronized on the monitor for the current object.

Does Java have multiple inheritance?

Java does not support multiple inheritence directly but it does thru the concept of interfaces.
We can make a class implement a number of interfaces if we want to achieve multiple inheritence type of functionality of C++.

How does exception handling work in Java?

1.It separates the working/functional code from the error-handling code by way of try-catch clauses.
2.It allows a clean path for error propagation. If the called method encounters a situation it can’t manage, it can throw an exception and let the calling method deal with it.
3.By enlisting the compiler to ensure that "exceptional" situations are anticipated and accounted for, it enforces powerful coding.
4.Exceptions are of two types: Compiler-enforced exceptions, or checked exceptions. Runtime exceptions, or unchecked exceptions. Compiler-enforced (checked) exceptions are instances of the Exception class or one of its subclasses — excluding the RuntimeException branch. The compiler expects all checked exceptions to be appropriately handled. Checked exceptions must be declared in the throws clause of the method throwing them — assuming, of course, they’re not being caught within that same method. The calling method must take care of these exceptions by either catching or declaring them in its throws clause. Thus, making an exception checked forces the us to pay heed to the possibility of it being thrown. An example of a checked exception is java.io.IOException. As the name suggests, it throws whenever an input/output operation is abnormally terminated.

Does Java have destructors?

Garbage collector does the job working in the background
Java does not have destructors; but it has finalizers that does a similar job.
the syntax is
public void finalize(){
}
if an object has a finalizer, the method is invoked before the system garbage collects the object

What does the "abstract" keyword mean in front of a method? A class?

Abstract keyword declares either a method or a class.
If a method has a abstract keyword in front of it,it is called abstract method.Abstract method hs no body.It has only arguments and return type.Abstract methods act as placeholder methods that are implemented in the subclasses.
Abstract classes can’t be instantiated.If a class is declared as abstract,no objects of that class can be created.If a class contains any abstract method it must be declared as abstract

Are Java constructors inherited ? If not, why not?

You cannot inherit a constructor. That is, you cannot create a instance of a subclass using a constructor of one of it’s superclasses. One of the main reasons is because you probably don’t want to overide the superclasses constructor, which would be possible if they were inherited. By giving the developer the ability to override a superclasses constructor you would erode the encapsulation abilities of the language.

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Linux Sed Interview Questions

  1. What is sed? - sed is stream editor, a Unix tool for working with streams of text data. See the awful truth about sed.
  2. How do you substitute strings with sed? - Use ’s/old/new’ command, so sed ’s/hello/goodbye/’ would substitute the occurrence of the word hello to goodbye.
  3. How do you inject text with sed? - & in the substitution string defines the pattern found in the search string. As an example, here’s us trying to find a word ‘hello’ and replacing it with ‘hello and how are you’:
    echo ‘hello there’ | sed ’s/^hello/& and how are you/’
  4. Can I find several patterns and refer to them in the replacement string? - Yes, use (pattern) and then refer to your patterns as \1, \2, \3 and so on.
  5. If the string is ‘old old old’ and I run ’s/old/new’, I get ‘new old old’ as the result. I need ‘new new new‘. - You forgot the global modifier, which would replace every occurrence of the pattern with the substitution. ’s/old/new/g‘ will work.
  6. But I want ‘old old new’ from the previous example. - Just use the numeric modifier saying you want the third occurrence to be replaced. ’s/old/new/3‘ will work.
  7. I wrote a rather complex sed script. How do I save and run it? - Assuming that your file is named myscript1.sed, you can invoke sed -f myscript1.sed.
  8. How do I delete trailing whitespaces from each line? - sed ’s/[ \t]*$//’ Here we’re replacing any occurrence of a space or a tab with nothing. Check sed one-liners for more examples.
  9. How do you print just a few first lines of the file? - sed 1q will give you just the first line, sed 10q the first 10 lines.
  10. How do you replace a pattern only if it’s found, so that it’s executed faster? - Nest the replacement statement: sed ‘/old/ s/old/new/g’ file.txt
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