Mysql interview questions and answers

BASIC of MYSQL/SQL : -

How to test database procedures and triggers?
Ans: The process of testing database procedures and triggers requires the knowledge of input parameters, output parameters, and EXEC statement. EXEC statement is helpful in running the procedure and observing the behavior of the tables. The EXEC command by itself can be used to execute a stored procedure: 
example: EXEC master.dbo.xp_msver

What is the database trigger, how to verify the trigger is fired or not and can you invoke trigger on demand?
Ans: Database trigger: A trigger is basically a stored procedure used to maintain the integrity of the data present in the database, executes automatically to response to a certain event of a table/view in a database.
To verify, the trigger is fired or not; use the query of common audit log will display the trigger of the data table.
Triggers can’t be invoked on demand, it invoked when table displays an action (insert, delete & update) defined on that particular table.

What are joins and mention different types of joins?
Ans: Join is used to combine two or more than two table and displays as a single set of data.

In SQL, the types of joins are,

• Inner Join
• Outer Join
• Left Join
• Right Join

In ORACLE/PLSQL, there are four different types of joins,

• Inner Join
• Left-Outer Join
• Right-Outer Join
• Full-Outer Join

What do you mean by index and explain different types of indexes?
Ans: Index is a database object, created on a column of the table to find data more quickly and efficiently (or we can say; it is used to speed up the searches/queries).
Different types of indexes are:

• B-Tree index
• Bitmap index
• Clustered index
• Covering index
• Non-unique index
• Unique index

What do you mean by DBMS?
Ans: A database is a structured set of data and a database management system (DBMS) is a computer software applications that create communication network between users, others applications and database to store, modify, and extract information from a database. Famous DBMSs are My SQL, Postgre SQL, Microsoft SQL Server, Oracle, SAP and IBM DB2.

What do you mean by DML?
Ans: A data manipulation language (DML) is a member of computer programming language facilitates us to select, insert, delete and update data in a database. DML is basically involved in Structured Query Language (SQL), facilitates users to retrieve and use data in a relational database.

What do you mean by DCL commands and explain the types of commands used by DCL?
Ans: Data Control Language (DCL) is a component of SQL, controls the use of data stored in a database. There are two types of DCL Commands,
Grant = The command facilitates users to access privilege to database
Revoke = Stop permitting users to access the database

Which SQL statements can be used in Database Testing? 
Ans: All DDL, DML, DCL, and TCL statements of SQL can be used in database testing,

DDL (Data Definition Language):
All statements related to this can be used in defining the database structure or schema.
Statements: CREATE, ALTER, DROP, TRUNCATE, COMMENT, and RENAME

DML (Data Manipulation Language):
All statements related to this can be used for managing data within schema objects.
Statements: SELECT, INSERT, UPDATE, DELETE, MERGE, CALL, EXPLAIN PLAN, and LOCK TABLE.

DCL (Data Control Language):
GRANT Statement and REVOKE Statement.

TCL (Transaction Control Language/Data Manipulation Language):
All statements related to this can be used to manage the changes made by DML statements. Using TCL, statements can be grouped together into logical transactions.
Statements: COMMIT, SAVEPOINT, ROLLBACK, and SET TRANSACTION.


What do you mean by white box testing and black box testing?
Ans: Black-box testing is a software testing technique to test the functional behavior of an application without getting into its internal structures of an application. Core application’s code/internal structure and programming knowledge is not required for black-box testing, but core knowledge of application functionality is required to test the application.
White Box Testing (WBT) is also known as Code-Based Testing or Structural Testing to test the internal structure of an software application. In case of white-box testing, tester should be having good understanding of internal structure of an application as well as good knowledge of programming skills to design test cases and internal structure of an application.


How to write a query to get the second largest value from a given column of a table?
Ans:
This is a query to get the second largest value from a given column of a table,

SELECT MAX(COLUMN_NAME) FROM TABLE_NAME
WHERE COLUMN_NAME <
(SELECT MAX(COLUMN_NAME) FROM TABLE_NAME)

For Example:-
To get the second largest marks from “Marks” column of a “Students” table
Select Max(Marks) from Students
Where Marks< (Select Max(Marks) from students)


How to write a query to get 10thhighest salary from an employee table?
Ans:

This is a query to get nth highest salary from an employee table,
SELECT *
FROM Employee Emp1
WHERE ( n ) = (
SELECT COUNT( DISTINCT ( Emp2.Employee_Salary ) )
FROM Employee Emp2
WHERE Emp2.Employee_Salary >= Emp1.Employee_Salary
)

Here, you can replace the n with any number. For example, if you want to get10th highest salary, then replace n with 10.

SELECT *
FROM Employee Emp1
WHERE (10) = (
SELECT COUNT( DISTINCT ( Emp2.Employee_Salary ) )
FROM Employee Emp2
WHERE Emp2.Employee_Salary >= Emp1.Employee_Salary
)

What does the RDBMS stands for and what are the major RDBMS used by SQL?
Ans: RDBMS stands for Relational Database Management Systems. Major RDBMS that are involved with SQL are Sybase, Oracle, Access, Ingres, Microsoft SQL server etc.

What is CMMI and describe different levels of CMM?
Ans: CMMI stands for Capability Maturity Model Integration is a process development training and evaluation model of various business processes in software engineering.
Five CMM Maturity levels for an organization are,
1) Initial: At this level, organizations don’t have any verified technique and environment, so that; usually organizations budget and schedule time go over.
2) Repeatable: At this level, organizations have basic techniques and guidelines to limit the cost and schedule time, capable in repeating the same in the next similar project.
3) Defined: At this level, all techniques are well organized and explained clear and standard form.
4) Managed: At this level, organizations are much more developed than Defined level. Here, techniques are contacted using statistical and other quantitive technique.
5) Optimizing: Organizations determinedly attempt to develop performance via modern technical progress.


What is the meaning of Record in a database?
Ans: In database, a record is the set of values/fields of a specific entity. For example; Office_Employee Record, Book Record etc…
===================================================

SELECT INTO statement - Select INTO statement used to create backup copy of table with selected field from existing table into new table.
e.g. 
select * into newBackupTableName from existingTable;

===================================================
QUESTION AND ANSWER : -

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. “mysqli” extension is an improvement over the old “mysql” extension. In fact, the “i” in mysqli stands for “improved”.
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. Explain advantages of InnoDB over MyISAM? - Row-level locking, transactions, foreign key constraints and crash recovery.
13. 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.
14. 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.
15. How do you control the max size of a HEAP table? - MySQL config variable max_heap_table_size.
16. What are CSV tables? - Those are the special tables, data for which is saved into comma-separated values files. They cannot be indexed.
17. Explain federated tables. - Introduced in MySQL 5.0, federated tables allow access to the tables located on other databases on other servers.
18. What is SERIAL data type in MySQL? - BIGINT NOT NULL PRIMARY KEY AUTO_INCREMENT
19. 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.
20. 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.
21. 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.
22. 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.
23. What happens if a table has one column defined as TIMESTAMP? - That field gets the current timestamp whenever the row gets altered.
24. 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.
25. 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.
26. 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.
27. 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.
28. 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.
29. 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.)
30. 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.
31. 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.
32. 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.
33. Which storage engines support transactions in MySQL? - Berkeley DB and InnoDB.
34. How do you convert to a different table type? - ALTER TABLE customers TYPE = InnoDB
35. How do you index just the first four bytes of the column? - ALTER TABLE customers ADD INDEX (business_name(4))
36. 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.
37. How do you prevent MySQL from caching a query? - SELECT SQL_NO_CACHE …
38. 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.
39. 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.
40. 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.
41. 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.
42. 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.
43. 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)
44. How would you select all the users, whose phone number is null? - SELECT user_name FROM users WHERE ISNULL(user_phonenumber);
45. 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
46. 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.
47. 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.
48. 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.
49. When would you use ORDER BY in DELETE statement? - When you’re not deleting by row ID. Such as in DELETE FROM _com_questions ORDER BY timestamp LIMIT 1. This will delete the most recently posted question in the table _com_questions.
50. How can you see all indexes defined for a table? - SHOW INDEX FROM _questions;
51. How would you change a column from VARCHAR(10) to VARCHAR(50)? - ALTER TABLE _questions CHANGE _content _CONTENT VARCHAR(50).
52. How would you delete a column? - ALTER TABLE _answers DROP answer_user_id.
53. How would you change a table to InnoDB? - ALTER TABLE _questions ENGINE innodb; 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? - VARCHARs with length less than 4 become CHARs
    CHARs with length more than 3 become VARCHARs. NOT NULL gets added to the columns declared as PRIMARY KEYs Default values such as NULL are specified for each column
54. How do I find out all databases starting with ‘tech’ to which I have access to? - SHOW DATABASES LIKE ‘tech%’;
55. How do you concatenate strings in MySQL? - CONCAT (string1, string2, string3)
56. How do you get a portion of a string? - SELECT SUBSTR(title, 1, 10)
57. 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.
58. How do you convert a string to UTF-8? - SELECT (_question USING utf8);
59. What do % and _ mean inside LIKE statement? - % corresponds to 0 or more characters, _ is exactly one character.
60. How do you get the month from a timestamp? - SELECT MONTH
61. How do you offload the time/date handling to MySQL? - SELECT DATE_FORMAT(_timestamp, ‘%Y-%m-%d’) from _questions; A similar TIME_FORMAT function deals with time.
62. How do you add three minutes to a date? - ADDDATE(_publication_date, INTERVAL 3 MINUTE)
63. 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.
64. 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.
65. 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’);
66. How are ENUMs and SETs represented internally? - As unique integers representing the powers of two, due to storage optimizations.
67. How do you show the currently running queries? - SHOW FULL PROCESSLIST;
68. How do you kill a MySQL query? - See the ID of it from the question above, then KILL id. You can separate multiple IDs by space.
69. I need to find out how many client connections were aborted by MySQL server. - It’s displayed in SHOW STATUS query, alternatively accessible via mysqladmin extended-status.
70. What does SET SQL_AUTO_IS_NULL =1 do? - you can find the last inserted row for a table that contains an AUTO_INCREMENT column by issuing WHERE auto_increment_column IS NULL
    
    1. What is normalization? Explain different levels of normalization?
    • Check out the article Q100139 from Microsoft knowledge base and of course, there’s much more information available in the net. It’ll be a good idea to get a hold of any RDBMS fundamentals text book, especially the one by C. J. Date. Most of the times, it will be okay if you can explain till third normal form.
    76. What is denormalization and when would you go for it?
    
    
    • As the name indicates, denormalization is the reverse process of normalization. It’s the controlled introduction of redundancy in to the database design. It helps improve the query performance as the number of joins could be reduced.
1. How do you implement one-to-one, one-to-many and many-to-many relationships while designing tables?
   One-to-One relationship can be implemented as a single table and rarely as two tables with primary and foreign key relationships. One-to-Many relationships are implemented by splitting the data into two tables with primary key and foreign key relationships. Many-to-Many relationships are implemented using a junction table with the keys from both the tables forming the composite primary key of the junction table. It will be a good idea to read up a database designing fundamentals text book.
2. What’s the difference between a primary key and a unique key?
   • Both primary key and unique enforce uniqueness of the column on which they are defined. But by default primary key creates a clustered index on the column, where are unique creates a nonclustered index by default. Another major difference is that, primary key doesn’t allow NULLs, but unique key allows one NULL only.
3. What are user defined datatypes and when you should go for them?
   • User defined datatypes let you extend the base SQL Server datatypes by providing a descriptive name, and format to the database. Take for example, in your database, there is a column called Flight_Num which appears in many tables. In all these tables it should be varchar(8). In this case you could create a user defined datatype called Flight_num_type of varchar(8) and use it across all your tables. See sp_addtype, sp_droptype in books online.
4. What is bit datatype and what’s the information that can be stored inside a bit column?
   • Bit datatype is used to store boolean information like 1 or 0 (true or false). Untill SQL Server 6.5 bit datatype could hold either a 1 or 0 and there was no support for NULL. But from SQL Server 7.0 onwards, bit datatype can represent a third state, which is NULL.
5. Define candidate key, alternate key, composite key.
   • A candidate key is one that can identify each row of a table uniquely. Generally a candidate key becomes the primary key of the table. If the table has more than one candidate key, one of them will become the primary key, and the rest are called alternate keys. A key formed by combining at least two or more columns is called composite key.
6. What are defaults? Is there a column to which a default can’t be bound?
   • A default is a value that will be used by a column, if no value is supplied to that column while inserting data. IDENTITY columns and timestamp columns can’t have defaults bound to them. See CREATE DEFAULT in books online.
7. What is a transaction and what are ACID properties?
   • A transaction is a logical unit of work in which, all the steps must be performed or none. ACID stands for Atomicity, Consistency, Isolation, Durability. These are the properties of a transaction. For more information and explanation of these properties, see SQL Server books online or any RDBMS fundamentals text book. Explain different isolation levels An isolation level determines the degree of isolation of data between concurrent transactions. The default SQL Server isolation level is Read Committed. Here are the other isolation levels (in the ascending order of isolation): Read Uncommitted, Read Committed, Repeatable Read, Serializable. See SQL Server books online for an explanation of the isolation levels. Be sure to read about SET TRANSACTION ISOLATION LEVEL, which lets you customize the isolation level at the connection level. Read Committed - A transaction operating at the Read Committed level cannot see changes made by other transactions until those transactions are committed. At this level of isolation, dirty reads are not possible but nonrepeatable reads and phantoms are possible. Read Uncommitted - A transaction operating at the Read Uncommitted level can see uncommitted changes made by other transactions. At this level of isolation, dirty reads, nonrepeatable reads, and phantoms are all possible. Repeatable Read - A transaction operating at the Repeatable Read level is guaranteed not to see any changes made by other transactions in values it has already read. At this level of isolation, dirty reads and nonrepeatable reads are not possible but phantoms are possible. Serializable - A transaction operating at the Serializable level guarantees that all concurrent transactions interact only in ways that produce the same effect as if each transaction were entirely executed one after the other. At this isolation level, dirty reads, nonrepeatable reads, and phantoms are not possible.
8. CREATE INDEX myIndex ON myTable(myColumn)What type of Index will get created after executing the above statement?
   • Non-clustered index. Important thing to note: By default a clustered index gets created on the primary key, unless specified otherwise.
9. What’s the maximum size of a row?
   • 8060 bytes. Don’t be surprised with questions like ‘what is the maximum number of columns per table’. 1024 columns per table. Check out SQL Server books online for the page titled: "Maximum Capacity Specifications". Explain Active/Active and Active/Passive cluster configurations Hopefully you have experience setting up cluster servers. But if you don’t, at least be familiar with the way clustering works and the two clusterning configurations Active/Active and Active/Passive. SQL Server books online has enough information on this topic and there is a good white paper available on Microsoft site. Explain the architecture of SQL Server This is a very important question and you better be able to answer it if consider yourself a DBA. SQL Server books online is the best place to read about SQL Server architecture. Read up the chapter dedicated to SQL Server Architecture.
10. What is lock escalation?
    • Lock escalation is the process of converting a lot of low level locks (like row locks, page locks) into higher level locks (like table locks). Every lock is a memory structure too many locks would mean, more memory being occupied by locks. To prevent this from happening, SQL Server escalates the many fine-grain locks to fewer coarse-grain locks. Lock escalation threshold was definable in SQL Server 6.5, but from SQL Server 7.0 onwards it’s dynamically managed by SQL Server.
11. What’s the difference between DELETE TABLE and TRUNCATE TABLE commands?
    • DELETE TABLE is a logged operation, so the deletion of each row gets logged in the transaction log, which makes it slow. TRUNCATE TABLE also deletes all the rows in a table, but it won’t log the deletion of each row, instead it logs the deallocation of the data pages of the table, which makes it faster. Of course, TRUNCATE TABLE can be rolled back. TRUNCATE TABLE is functionally identical to DELETE statement with no WHERE clause: both remove all rows in the table. But TRUNCATE TABLE is faster and uses fewer system and transaction log resources than DELETE. The DELETE statement removes rows one at a time and records an entry in the transaction log for each deleted row. TRUNCATE TABLE removes the data by deallocating the data pages used to store the table’s data, and only the page deallocations are recorded in the transaction log. TRUNCATE TABLE removes all rows from a table, but the table structure and its columns, constraints, indexes and so on remain. The counter used by an identity for new rows is reset to the seed for the column. If you want to retain the identity counter, use DELETE instead. If you want to remove table definition and its data, use the DROP TABLE statement. You cannot use TRUNCATE TABLE on a table referenced by a FOREIGN KEY constraint; instead, use DELETE statement without a WHERE clause. Because TRUNCATE TABLE is not logged, it cannot activate a trigger. TRUNCATE TABLE may not be used on tables participating in an indexed view
12. Explain the storage models of OLAP
    • Check out MOLAP, ROLAP and HOLAP in SQL Server books online for more infomation.
13. What are the new features introduced in SQL Server 2000 (or the latest release of SQL Server at the time of your interview)? What changed between the previous version of SQL Server and the current version?
    • This question is generally asked to see how current is your knowledge. Generally there is a section in the beginning of the books online titled "What’s New", which has all such information. Of course, reading just that is not enough, you should have tried those things to better answer the questions. Also check out the section titled "Backward Compatibility" in books online which talks about the changes that have taken place in the new version.
14. What are constraints? Explain different types of constraints.
    • Constraints enable the RDBMS enforce the integrity of the database automatically, without needing you to create triggers, rule or defaults. Types of constraints: NOT NULL, CHECK, UNIQUE, PRIMARY KEY, FOREIGN KEY. For an explanation of these constraints see books online for the pages titled: "Constraints" and "CREATE TABLE", "ALTER TABLE"
15. What is an index? What are the types of indexes? How many clustered indexes can be created on a table? I create a separate index on each column of a table. What are the advantages and disadvantages of this approach?
    • Indexes in SQL Server are similar to the indexes in books. They help SQL Server retrieve the data quicker. Indexes are of two types. Clustered indexes and non-clustered indexes. When you create a clustered index on a table, all the rows in the table are stored in the order of the clustered index key. So, there can be only one clustered index per table. Non-clustered indexes have their own storage separate from the table data storage. Non-clustered indexes are stored as B-tree structures (so do clustered indexes), with the leaf level nodes having the index key and it’s row locater. The row located could be the RID or the Clustered index key, depending up on the absence or presence of clustered index on the table. If you create an index on each column of a table, it improves the query performance, as the query optimizer can choose from all the existing indexes to come up with an efficient execution plan. At the same t ime, data modification operations (such as INSERT, UPDATE, DELETE) will become slow, as every time data changes in the table, all the indexes need to be updated. Another disadvantage is that, indexes need disk space, the more indexes you have, more disk space is used.
16. What is RAID and what are different types of RAID configurations?
    • RAID stands for Redundant Array of Inexpensive Disks, used to provide fault tolerance to database servers. There are six RAID levels 0 through 5 offering different levels of performance, fault tolerance. MSDN has some information about RAID levels and for detailed information, check out the RAID advisory board’s homepage
17. What are the steps you will take to improve performance of a poor performing query?
    • This is a very open ended question and there could be a lot of reasons behind the poor performance of a query. But some general issues that you could talk about would be: No indexes, table scans, missing or out of date statistics, blocking, excess recompilations of stored procedures, procedures and triggers without SET NOCOUNT ON, poorly written query with unnecessarily complicated joins, too much normalization, excess usage of cursors and temporary tables. Some of the tools/ways that help you troubleshooting performance problems are: SET SHOWPLAN_ALL ON, SET SHOWPLAN_TEXT ON, SET STATISTICS IO ON, SQL Server Profiler, Windows NT /2000 Performance monitor, Graphical execution plan in Query Analyzer. Download the white paper on performance tuning SQL Server from Microsoft web site. Don’t forget to check out sql-server-performance.com
18. What are the steps you will take, if you are tasked with securing an SQL Server?
    • Again this is another open ended question. Here are some things you could talk about: Preferring NT authentication, using server, databse and application roles to control access to the data, securing the physical database files using NTFS permissions, using an unguessable SA password, restricting physical access to the SQL Server, renaming the Administrator account on the SQL Server computer, disabling the Guest account, enabling auditing, using multiprotocol encryption, setting up SSL, setting up firewalls, isolating SQL Server from the web server etc. Read the white paper on SQL Server security from Microsoft website. Also check out My SQL Server security best practices
19. What is a deadlock and what is a live lock? How will you go about resolving deadlocks?
    • Deadlock is a situation when two processes, each having a lock on one piece of data, attempt to acquire a lock on the other’s piece. Each process would wait indefinitely for the other to release the lock, unless one of the user processes is terminated. SQL Server detects deadlocks and terminates one user’s process. A livelock is one, where a request for an exclusive lock is repeatedly denied because a series of overlapping shared locks keeps interfering. SQL Server detects the situation after four denials and refuses further shared locks. A livelock also occurs when read transactions monopolize a table or page, forcing a write transaction to wait indefinitely. Check out SET DEADLOCK_PRIORITY and "Minimizing Deadlocks" in SQL Server books online. Also check out the article Q169960 from Microsoft knowledge base.
20. What is blocking and how would you troubleshoot it?
    • Blocking happens when one connection from an application holds a lock and a second connection requires a conflicting lock type. This forces the second connection to wait, blocked on the first. Read up the following topics in SQL Server books online: Understanding and avoiding blocking, Coding efficient transactions. Explain CREATE DATABASE syntax Many of us are used to creating databases from the Enterprise Manager or by just issuing the command: CREATE DATABAE MyDB.
21. But what if you have to create a database with two filegroups, one on drive C and the other on drive D with log on drive E with an initial size of 600 MB and with a growth factor of 15%?
    • That’s why being a DBA you should be familiar with the CREATE DATABASE syntax. Check out SQL Server books online for more information.
22. How to restart SQL Server in single user mode? How to start SQL Server in minimal configuration mode?
    • SQL Server can be started from command line, using the SQLSERVR.EXE. This EXE has some very important parameters with which a DBA should be familiar with. -m is used for starting SQL Server in single user mode and -f is used to start the SQL Server in minimal configuration mode. Check out SQL Server books online for more parameters and their explanations.
23. As a part of your job, what are the DBCC commands that you commonly use for database maintenance?
    • DBCC CHECKDB, DBCC CHECKTABLE, DBCC CHECKCATALOG, DBCC CHECKALLOC, DBCC SHOWCONTIG, DBCC SHRINKDATABASE, DBCC SHRINKFILE etc. But there are a whole load of DBCC commands which are very useful for DBAs. Check out SQL Server books online for more information.
24. What are statistics, under what circumstances they go out of date, how do you update them?
    • Statistics determine the selectivity of the indexes. If an indexed column has unique values then the selectivity of that index is more, as opposed to an index with non-unique values. Query optimizer uses these indexes in determining whether to choose an index or not while executing a query. Some situations under which you should update statistics: 1) If there is significant change in the key values in the index 2) If a large amount of data in an indexed column has been added, changed, or removed (that is, if the distribution of key values has changed), or the table has been truncated using the TRUNCATE TABLE statement and then repopulated 3) Database is upgraded from a previous version. Look up SQL Server books online for the following commands: UPDATE STATISTICS, STATS_DATE, DBCC SHOW_STATISTICS, CREATE STATISTICS, DROP STATISTICS, sp_autostats, sp_createstats, sp_updatestats
25. What are the different ways of moving data/databases between servers and databases in SQL Server?
    • There are lots of options available, you have to choose your option depending upon your requirements. Some of the options you have are: BACKUP/RESTORE, dettaching and attaching databases, replication, DTS, BCP, logshipping, INSERT…SELECT, SELECT…INTO, creating INSERT scripts to generate data.
26. Explain different types of BACKUPs avaialabe in SQL Server? Given a particular scenario, how would you go about choosing a backup plan?
    • Types of backups you can create in SQL Sever 7.0+ are Full database backup, differential database backup, transaction log backup, filegroup backup. Check out the BACKUP and RESTORE commands in SQL Server books online. Be prepared to write the commands in your interview. Books online also has information on detailed backup/restore architecture and when one should go for a particular kind of backup.
27. What is database replication? What are the different types of replication you can set up in SQL Server?
    • Replication is the process of copying/moving data between databases on the same or different servers. SQL Server supports the following types of replication scenarios: � Snapshot replication � Transactional replication (with immediate updating subscribers, with queued updating subscribers) � Merge replication See SQL Server books online for indepth coverage on replication. Be prepared to explain how different replication agents function, what are the main system tables used in replication etc.
28. How to determine the service pack currently installed on SQL Server?
    • The global variable @@Version stores the build number of the sqlservr.exe, which is used to determine the service pack installed. To know more about this process visit SQL Server service packs and versions.
29. What are cursors? Explain different types of cursors. What are the disadvantages of cursors? How can you avoid cursors?
    • Cursors allow row-by-row processing of the resultsets. Types of cursors: Static, Dynamic, Forward-only, Keyset-driven. See books online for more information. Disadvantages of cursors: Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one roundtrip, however large the resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Further, there are restrictions on the SELECT statements that can be used with some types of cursors. Most of the times, set based operations can be used instead of cursors. Here is an example: If you have to give a flat hike to your employees using the following criteria: Salary between 30000 and 40000 — 5000 hike Salary between 40000 and 55000 — 7000 hike Salary between 55000 and 65000 — 9000 hike. In this situation many developers tend to use a cursor, determine each employee’s salary and update his salary according to the above formula. But the same can be achieved by multiple update statements or can be combined in a single UPDATE statement as shown below:
30. UPDATE tbl_emp SET salary = CASE WHEN salary BETWEEN 30000 AND 40000 THEN salary + 5000 WHEN salary BETWEEN 40000 AND 55000 THEN salary + 7000 WHEN salary BETWEEN 55000 AND 65000 THEN salary + 10000 END
31. Another situation in which developers tend to use cursors: You need to call a stored procedure when a column in a particular row meets certain condition. You don’t have to use cursors for this. This can be achieved using WHILE loop, as long as there is a unique key to identify each row. For examples of using WHILE loop for row by row processing, check out the ‘My code library’ section of my site or search for WHILE. Write down the general syntax for a SELECT statements covering all the options. Here’s the basic syntax: (Also checkout SELECT in books online for advanced syntax).
32. SELECT select_list [INTO new_table_] FROM table_source [WHERE search_condition] [GROUP BY group_by_expression] [HAVING search_condition] [ORDER BY order_expression [ASC | DESC] ]
33. What is a join and explain different types of joins.
    • Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table. Types of joins: INNER JOINs, OUTER JOINs, CROSS JOINs. OUTER JOINs are further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS. For more information see pages from books online titled: "Join Fundamentals" and "Using Joins".
34. Can you have a nested transaction?
    • Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT
35. What is an extended stored procedure? Can you instantiate a COM object by using T-SQL?
    • An extended stored procedure is a function within a DLL (written in a programming language like C, C++ using Open Data Services (ODS) API) that can be called from T-SQL, just the way we call normal stored procedures using the EXEC statement. See books online to learn how to create extended stored procedures and how to add them to SQL Server. Yes, you can instantiate a COM (written in languages like VB, VC++) object from T-SQL by using sp_OACreate stored procedure. Also see books online for sp_OAMethod, sp_OAGetProperty, sp_OASetProperty, sp_OADestroy. For an example of creating a COM object in VB and calling it from T-SQL, see ‘My code library’ section of this site.
36. What is the system function to get the current user’s user id?
    • USER_ID(). Also check out other system functions like USER_NAME(), SYSTEM_USER, SESSION_USER, CURRENT_USER, USER, SUSER_SID(), HOST_NAME().
37. What are triggers? How many triggers you can have on a table? How to invoke a trigger on demand?
    • Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table. In SQL Server 6.5 you could define only 3 triggers per table, one for INSERT, one for UPDATE and one for DELETE. From SQL Server 7.0 onwards, this restriction is gone, and you could create multiple triggers per each action. But in 7.0 there’s no way to control the order in which the triggers fire. In SQL Server 2000 you could specify which trigger fires first or fires last using sp_settriggerorder. Triggers can’t be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens on the table on which they are defined. Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster. Till SQL Server 7.0, triggers fire only after the data modification operation happens. So in a way, they are called post triggers. But in SQL Server 2000 you could create pre triggers also. Search SQL Server 2000 books online for INSTEAD OF triggers. Also check out books online for ‘inserted table’, ‘deleted table’ and COLUMNS_UPDATED()
38. There is a trigger defined for INSERT operations on a table, in an OLTP system. The trigger is written to instantiate a COM object and pass the newly insterted rows to it for some custom processing. What do you think of this implementation? Can this be implemented better?
    • Instantiating COM objects is a time consuming process and since you are doing it from within a trigger, it slows down the data insertion process. Same is the case with sending emails from triggers. This scenario can be better implemented by logging all the necessary data into a separate table, and have a job which periodically checks this table and does the needful.
39. What is a self join? Explain it with an example.
    • Self join is just like any other join, except that two instances of the same table will be joined in the query. Here is an example: Employees table which contains rows for normal employees as well as managers. So, to find out the managers of all the employees, you need a self join.
40. CREATE TABLE emp ( empid int, mgrid int, empname char(10) )
41. INSERT emp SELECT 1,2,’Vyas’ INSERT emp SELECT 2,3,’Mohan’ INSERT emp SELECT 3,NULL,’Shobha’ INSERT emp SELECT 4,2,’Shridhar’ INSERT emp SELECT 5,2,’Sourabh’
42. SELECT t1.empname [Employee], t2.empname [Manager] FROM emp t1, emp t2 WHERE t1.mgrid = t2.empid Here’s an advanced query using a LEFT OUTER JOIN that even returns the employees without managers (super bosses)
43. SELECT t1.empname [Employee], COALESCE(t2.empname, ‘No manager’) [Manager] FROM emp t1 LEFT OUTER JOIN emp t2 ON t1.mgrid = t2.empid