SQL Interview Questions

1.Which TCP/IP port does SQL Server run on? How can it be changed?

A.SQL Server runs on port 1433. It can be changed from the Network Utility TCP/IP properties.

2.What are the difference between clustered and a non-clustered index?

A.A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore table can have only one clustered index. The leaf nodes of a clustered index contain the data pages. A non clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows.

3.What are the different index configurations a table can have?

A.A table can have one of the following index configurations: No indexes, A clustered index, A clustered index and many nonclustered indexes, A nonclustered index and Many nonclustered indexes

4.What are different types of Collation Sensitivity?

A.Case sensitivity - A and a, B and b, etc, Accent sensitivity, Kana Sensitivity - When Japanese kana characters Hiragana and Katakana are treated differently, it is called Kana sensitive, Width sensitivity - A single-byte character (half-width) and the same character represented as a double-byte character (full-width) are treated differently than it is width sensitive.

5.What is OLTP (Online Transaction Processing)?

A.In OLTP - online transaction processing systems relational database design use the discipline of data modeling and generally follow the Codd rules of data normalization in order to ensure absolute data integrity. Using these rules complex information is broken down into its most simple structures (a table) where all of the individual atomic level elements relate to each other and satisfy the normalization rules.

6.Whats the difference between a primary key and a unique key?

A.Both primary key and unique key enforces 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 doesnt allow NULLs, but unique key allows one NULL only.

7.What is meant by DELETE?

A.Delete command removes the rows from a table based on the condition that we provide with a WHERE clause. Truncate will actually remove all the rows from a table and there will be no data in the table after we run the truncate command.

8.What is meant by TRUNCATE?

A.TRUNCATE is faster and uses fewer system and transaction log resources than DELETE.TRUNCATE removes all rows from a table, but the table structure, its columns, constraints, indexes and so on, remains. The counter used by an identity for new rows is reset to the seed for the column.

9.When is the use of UPDATE_STATISTICS command?

A.This command is basically used when a large processing of data has occurred. If a large amount of deletions any modification or Bulk Copy into the tables has occurred, it has to update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.

10.What is the difference between a HAVING CLAUSE and a WHERE CLAUSE?

A.They specify a search condition for a group or an aggregate. But the difference is that HAVING can be used only with the SELECT statement. HAVING is typically used in a GROUP BY clause. When GROUP BY is not used, HAVING behaves like a WHERE clause. Having Clause is basically used only with the GROUP BY function in a query whereas WHERE Clause is applied to each row before they are part of the GROUP BY function in a query.

11.What are the properties of Sub-Queries?

A.A sub-query must be enclosed in the parenthesis, A sub-query must be put in the right hand of the comparison operator, and A sub-query cannot contain an ORDER-BY clause, A query can contain more than one sub-query.

12.What is SQL Profiler?

A.SQL Profiler is a graphical tool that allows system administrators to monitor events in an instance of Microsoft SQL Server. You can capture and save data about each event to a file or SQL Server table to analyze later. For example, you can monitor a production environment to see which stored procedures are hampering performances by executing too slowly. Use SQL Profiler to monitor only the events in which you are interested. If traces are becoming too large, you can filter them based on the information you want, so that only a subset of the event data is collected. Monitoring too many events adds overhead to the server and the monitoring process and can cause the trace file or trace table to grow very large, especially when the monitoring process takes place over a long period of time.

13.What are the authentication modes in SQL Server? How can it be changed?

A.Windows mode and Mixed Mode - SQL and Windows. To change authentication mode in SQL Server click Start, Programs, Microsoft SQL Server and click SQL Enterprise Manager to run SQL Enterprise Manager from the Microsoft SQL Server program group. Select the server then from the Tools menu select SQL Server Configuration Properties, and choose the Security page.

14.What is SQL Server Agent?

A.SQL Server agent plays an important role in the day-to-day tasks of a database administrator (DBA). It is often overlooked as one of the main tools for SQL Server management. Its purpose is to ease the implementation of tasks for the DBA, with its full- function scheduling engine, which allows you to schedule your own jobs and scripts.

15.Can a stored procedure call itself or recursive stored procedure? How much level SP nesting is possible?

A.Yes. Because Transact-SQL supports recursion, you can write stored procedures that call themselves. Recursion can be defined as a method of problem solving wherein the solution is arrived at by repetitively applying it to subsets of the problem. A common application of recursive logic is to perform numeric computations that lend themselves to repetitive evaluation by the same processing steps. Stored procedures are nested when one stored procedure calls another or executes managed code by referencing a CLR routine, type, or aggregate. You can nest stored procedures and managed code references up to 32 levels.

16.What is Log Shipping?

A.Log shipping is the process of automating the backup of database and transaction log files on a production SQL server, and then restoring them onto a standby server. Enterprise Editions only supports log shipping. In log shipping the transactional log file from one server is automatically updated into the backup database on the other server. If one server fails, the other server will have the same db and can be used this as the Disaster Recovery plan. The key feature of log shipping is that it will automatically backup transaction logs throughout the day and automatically restore them on the standby server at defined interval.

17.What does it mean to have QUOTED_IDENTIFIER ON? What are the implications of having it OFF?

A.When SET QUOTED_IDENTIFIER is ON, identifiers can be delimited by double quotation marks, and literals must be delimited by single quotation marks. When SET QUOTED_IDENTIFIER is OFF, identifiers cannot be quoted and must follow all Transact-SQL rules for identifiers.

18.What is a Local temporary table?

A.A local temporary table exists only for the duration of a connection or, if defined inside a compound statement, for the duration of the compound statement.

19.What is a Global temporary table?

A.A global temporary table remains in the database permanently, but the rows exist only within a given connection. When connection is closed, the data in the global temporary table disappears. However, the table definition remains with the database for access when database is opened next time.

20.What is the STUFF function and how does it differ from the REPLACE function?

A.STUFF function is used to overwrite existing characters. Using this syntax, STUFF (string_expression, start, length, replacement_characters), string_expression is the string that will have characters substituted, start is the starting position, length is the number of characters in the string that are substituted, and replacement_characters are the new characters interjected into the string. REPLACE function to replace existing characters of all occurrences. Using the syntax REPLACE (string_expression, search_string, replacement_string), where every incidence of search_string found in the string_expression will be replaced with replacement_string.

21.What is PRIMARY KEY?

A.A PRIMARY KEY constraint is a unique identifier for a row within a database table. Every table should have a primary key constraint to uniquely identify each row and only one primary key constraint can be created for each table. The primary key constraints are used to enforce entity integrity.

22.What is UNIQUE KEY constraint?

A.A UNIQUE constraint enforces the uniqueness of the values in a set of columns, so no duplicate values are entered. The unique key constraints are used to enforce entity integrity as the primary key constraints.

23.What is FOREIGN KEY?

A.A FOREIGN KEY constraint prevents any actions that would destroy links between tables with the corresponding data values. A foreign key in one table points to a primary key in another table. Foreign keys prevent actions that would leave rows with foreign key values when there are no primary keys with that value. The foreign key constraints are used to enforce referential integrity.

24.What is CHECK Constraint?

A.A CHECK constraint is used to limit the values that can be placed in a column. The check constraints are used to enforce domain integrity.

25.What is NOT NULL Constraint?

A.A NOT NULL constraint enforces that the column will not accept null values. The not null constraints are used to enforce domain integrity, as the check constraints.

26.How to get @@ERROR and @@ROWCOUNT at the same time?

A.If @@Rowcount is checked after Error checking statement then it will have 0 as the value of @@Recordcount as it would have been reset. And if @@Recordcount is checked before the error-checking statement then @@Error would get reset. To get @@error and @@rowcount at the same time do both in same statement and store them in local variable. SELECT @RC = @@ROWCOUNT, @ER = @@ERROR

27.What is a Scheduled Jobs or What is a Scheduled Tasks?

A.Scheduled tasks let user automate processes that run on regular or predictable cycles. User can schedule administrative tasks, such as cube processing, to run during times of slow business activity. User can also determine the order in which tasks run by creating job steps within a SQL Server Agent job. E.g. back up database, Update Stats of Tables. Job steps give user control over flow of execution. If one job fails, user can configure SQL Server Agent to continue to run the remaining tasks or to stop execution.

28.What are the advantages of using Stored Procedures?

A.Stored procedure can reduced network traffic and latency, boosting application performance, Stored procedure execution plans can be reused, staying cached in SQL Servers memory, reducing server overhead, Stored procedures help promote code reuse.

29.What is a table called, if it has neither Cluster nor Non-cluster Index? What is it used for?

A.Unindexed table or Heap. Microsoft Press Books and Book on Line (BOL) refers it as Heap. A heap is a table that does not have a clustered index and, therefore, the pages are not linked by pointers. The IAM pages are the only structures that link the pages in a table together. Unindexed tables are good for fast storing of data. Many times it is better to drop all indexes from table and then do bulk of inserts and to restore those indexes after that.

30.Can SQL Servers linked to other servers like Oracle?

A.SQL Server can be linked to any server provided it has OLE-DB provider from Microsoft to allow a link. E.g. Oracle has an OLE-DB provider for oracle that Microsoft provides to add it as linked server to SQL Server group.

31.What is BCP? When does it used?

A.BulkCopy is a tool used to copy huge amount of data from tables and views. BCP does not copy the structures same as source to destination. BULK INSERT command helps to import a data file into a database table or view in a user-specified format.

32.How to implement one-to-one, one-to-many and many-to-many relationships while designing tables?

A.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.

33.What is an execution plan? When would you use it? How would you view the execution plan?

A.An execution plan is basically a road map that graphically or textually shows the data retrieval methods chosen by the SQL Server query optimizer for a stored procedure or ad- hoc query and is a very useful tool for a developer to understand the performance characteristics of a query or stored procedure since the plan is the one that SQL Server will place in its cache and use to execute the stored procedure or query. From within Query Analyzer is an option called "Show Execution Plan" (located on the Query drop-down menu). If this option is turned on it will display query execution plan in separate window when query is ran again.

34.Get all employee details from the employee table

A. Select * from employee

35.Get First_Name,Last_Name from employee table

A.Select first_name, Last_Name from employee

36.Get First_Name from employee table using alias name �Employee Name�

A. Select first_name Employee Name from employee

37.Get First_Name from employee table in upper case

A.Select upper(FIRST_NAME) from EMPLOYEE

38.Get First_Name from employee table in lower case

A.Select lower(FIRST_NAME) from EMPLOYEE.

39.Get unique DEPARTMENT from employee table

A.select distinct DEPARTMENT from EMPLOYEE.

40.Get FIRST_NAME from employee table after removing white spaces from right side?

A.select RTRIM(FIRST_NAME) from employee.

41.Get FIRST_NAME from employee table after removing white spaces from left side?

A.select LTRIM(FIRST_NAME) from employee.

42.Get all employee details from the employee table order by First_Name Ascending

A.Select * from employee order by FIRST_NAME asc

43.Get all employee details from the employee table order by First_Name descending

A.Select * from employee order by FIRST_NAME desc

44.Get all employee details from the employee table order by First_Name Ascending and Salary descending

A.Select * from employee order by FIRST_NAME asc,SALARY desc

45.Get employee details from employee table whose Salary greater than 600000

A.Select * from EMPLOYEE where Salary > 600000

46.Get employee details from employee table whose Salary less than 800000

A.Select * from EMPLOYEE where Salary < 800000

47.Get employee details from employee table whose Salary between 500000 and 800000

A.Select * from EMPLOYEE where Salary between 500000 and 800000

48.Get department,total salary with respect to a department from employee table.

A.Select DEPARTMENT,sum(SALARY) Total_Salary from employee group by department

49.Get department,total salary with respect to a department from employee table order by total salary descending

A.Select DEPARTMENT,sum(SALARY) Total_Salary from employee group by DEPARTMENT order by Total_Salary descending

50.Get department,no of employees in a department,total salary with respect to a department from employee table order by total salary descending

A.Select DEPARTMENT,count(FIRST_NAME),sum(SALARY) Total_Salary from employee group by DEPARTMENT order by Total_Salary descending

51.Get department wise average salary from employee table order by salary ascending

A.select DEPARTMENT,avg(SALARY) AvgSalary from employee group by DEPARTMENT order by AvgSalary asc

52.Get department wise maximum salary from employee table order by salary ascending

A.select DEPARTMENT,max(SALARY) MaxSalary from employee group by DEPARTMENT order by MaxSalary asc

53.Get department wise minimum salary from employee table order by salary ascending

A.select DEPARTMENT,min(SALARY) MinSalary from employee group by DEPARTMENT order by MinSalary asc

54.Select department,total salary with respect to a department from employee table where total salary greater than 800000 order by Total_Salary descending

A.Select DEPARTMENT,sum(SALARY) Total_Salary from employee group by DEPARTMENT having sum(SALARY) > 800000 order by Total_Salary desc

55.Select first_name, incentive amount from employee and incentives table for those employees who have incentives

A.Select FIRST_NAME,INCENTIVE_AMOUNT from employee a inner join incentives B on A.EMPLOYEE_ID = B.EMPLOYEE_REF_ID

56.Select first_name, incentive amount from employee and incentives table for those employees who have incentives and incentive amount greater than 3000

A.Select FIRST_NAME,INCENTIVE_AMOUNT from employee a inner join incentives B on A.EMPLOYEE_ID = B.EMPLOYEE_REF_ID and INCENTIVE_AMOUNT > 3000

57.Select first_name, incentive amount from employee and incentives table for all employes even if they didnt get incentives

A.Select FIRST_NAME,INCENTIVE_AMOUNT from employee a left join incentives B on A.EMPLOYEE_ID = B.EMPLOYEE_REF_ID

58.Select First_Name,LAST_NAME from employee table as separate rows

A.select FIRST_NAME from EMPLOYEE union select LAST_NAME from EMPLOYEE

59.Select employee details from employee table if data exists in incentive table

A.select * from EMPLOYEE where exists (select * from INCENTIVES)

60.Select employee details from employee table if data exists in incentive table

A.select * from EMPLOYEE where exists (select * from INCENTIVES)

61. Delete employee data from employee table who got incentives in incentive table

A.delete from EMPLOYEE where EMPLOYEE_ID in (select EMPLOYEE_REF_ID from INCENTIVES)

62. Delete employee data from employee table who got incentives in incentive table

A.delete from EMPLOYEE where EMPLOYEE_ID in (select EMPLOYEE_REF_ID from INCENTIVES)

63.Select Last Name from employee table which contain only numbers

A.Select * from EMPLOYEE where lower(LAST_NAME) = upper(LAST_NAME)

64.Write syntax to delete table employee

A.DROP table employee;

65.Write syntax to set EMPLOYEE_ID as primary key in employee table

A.ALTER TABLE EMPLOYEE add CONSTRAINT EMPLOYEE_PK PRIMARY KEY(EMPLOYEE_ID)

66. Write syntax to set 2 fields(EMPLOYEE_ID,FIRST_NAME) as primary key in employee table

A.ALTER TABLE EMPLOYEE add CONSTRAINT EMPLOYEE_PK PRIMARY KEY(EMPLOYEE_ID,FIRST_NAME)

67.Write syntax to drop primary key on employee table

A.Alter TABLE EMPLOYEE drop CONSTRAINT EMPLOYEE_PK;

68.Write Sql Syntax to create EMPLOYEE_REF_ID in INCENTIVES table as foreign key with respect to EMPLOYEE_ID in employee table

A.ALTER TABLE INCENTIVES ADD CONSTRAINT INCENTIVES_FK FOREIGN KEY (EMPLOYEE_REF_ID) REFERENCES EMPLOYEE(EMPLOYEE_ID)ALTER TABLE INCENTIVES ADD CONSTRAINT INCENTIVES_FK FOREIGN KEY (EMPLOYEE_REF_ID) REFERENCES EMPLOYEE(EMPLOYEE_ID)

69.Write SQL to drop foreign key on employee table

A.ALTER TABLE INCENTIVES drop CONSTRAINT INCENTIVES_FK;

70.Write SQL to create Orcale Sequence

A.CREATE SEQUENCE EMPLOYEE_ID_SEQ START WITH 0 NOMAXVALUE MINVALUE 0 NOCYCLE NOCACHE NOORDER;

71. What is SQL Injection ?

A.SQL Injection is one of the the techniques uses by hackers to hack a website by injecting SQL commands in data fields.

72.What is a subselect? Is it different from a nested select?

A.A subselect is a select which works in conjunction with another select. A nested select is a kind of subselect where the inner select passes to the where criteria for the outer select.

73.What is the difference between group by and order by?

A.Group by controls the presentation of the rows, order by controls the presentation of the columns for the results of the SELECT statement.

74.What keyword does an SQL SELECT statement use for a string search?

A.The LIKE keyword allows for string searches. The % sign is used as a wildcard.

75.What are some SQL aggregates and other built-in functions?

A.The common aggregate, built-in functions are AVG, SUM, MIN, MAX, COUNT and DISTINCT.

76.Explain the EXPLAIN statement?

A.The explain statement provides information about the optimizers choice of access path of the SQL.

77.What is referential integrity?

A.Referential integrity refers to the consistency that must be maintained between primary and foreign keys, i.e. every foreign key value must have a corresponding primary key value.

78.What is a NULL value? What are the pros and cons of using NULLS?

A.A NULL value takes up one byte of storage and indicates that a value is not present as opposed to a space or zero value. Its the DB2 equivalent of TBD on an organizational chart and often correctly portrays a business situation.Unfortunately, it requires extra coding for an application program to handle this situation.

79.What is a synonym? How is it used?

A.A synonym is used to reference a table or view by another name.The other name can then be written in the application code pointing to test tables in the development stage and to production entities when the code is migrated.The synonym is linked to the AUTHID that created it.

80.What is an alias and how does it differ from a synonym?

A.An alias is an alternative to a synonym, designed for a distributed environment to avoid having to use the location qualifier of a table or view. The alias is not dropped when the table is dropped.

81.When can an insert of a new primary key value threaten referential integrity?

A.Never. New primary key values are not a problem. However, the values of foreign key inserts must have corresponding primary key values in their related tables. And updates of primary key values may require changes in foreign key values to maintain referential integrity.

82.What is the difference between static and dynamic SQL?

A.Static SQL is hard-coded in a program when the programmer knows the statements to be executed. For dynamic SQL the program must dynamically allocate memory to receive the query results.

83.Static SQL is hard-coded in a program when the programmer knows the statements to be executed. For dynamic SQL the program must dynamically allocate memory to receive the query results.

A.Any subselect can be rewritten as a join, but not vice versa. Joins are usually more efficient as join rows can be returned immediately, subselects require a temporary work area for inner selects results while processing the outer select.

84.What is the difference between IN subselects and EXISTS subselect?

A.If there is an index on the attributes tested an IN is more efficient since DB2 uses the index for the IN. (IN for index is the mnemonic).

85.What is a Cartesian product?

A.A Cartesian product results from a faulty query. It is a row in the results for every combination in the join tables.

86.What is a tuple?

A.A tuple is an instance of data within a relational database.

87.What is the difference between static and dynamic SQL?

A.Static SQL is compiled and optimized prior to its execution; dynamic is compiled and optimized during execution.

88.What is SQL ?

A.Structured Query Language(SQL) is a language designed specifically for communicating with databases. SQL is an ANSI (American National Standards Institute) standard .

89.What are the different type of Sqls?

A. DDL � Data Definition Language, DML � Data Manipulation Language and DCL � Data Control Language

90.what is a field in a database ?

A.A field is an area within a record reserved for a specific piece of data. Examples: Employee Name , Employee ID etc

91.What is a Record in a database ?

A.A record is the collection of values / fields of a specific entity: i.e. a Employee , Salary etc

92.What is a Table in a database ?

A.A table is a collection of records of a specific type. For example, employee table , salary table etc .

93.What is a database transaction?

A.Frequently asked SQL Interview Questions, Database transaction take DB from one consistent state to another. At the end of the transaction the system must be in the prior state if transaction fails or the status of the system should reflect the successful completion if the transaction goes through.

94.What are properties of a transaction?

A.Frequently asked SQL Interview Questions Properties of the transaction can be summarized as ACID Properties.

95.What is a Database Lock?

A.Database lock tell a transaction if the data item in questions is currently being used by other transactions.

96.What is a SQL Composite Primary Key

A.A Composite primary key is a set of columns whose values uniquely identify every row in a table. For example , in the table given above , if "Employee_ID" and "Employee Name" together uniquely identifies a row its called a Composite Primary Key . In this case , both the columns will be represented as primary key .

97.Define SQL Update Statement?

A.SQL Update is used to update data in a row or set of rows specified in the filter condition . The basic format of an SQL UPDATE statement is ,Update command followed by table to be updated and SET command followed by column names and their new values followed by filter condition that determines which rows should be updated

98.Define SQL Delete Statement?

A.SQL Delete is used to delete a row or set of rows specified in the filter condition .The basic format of an SQL DELETE statement is ,DELETE FROM command followed by table name followed by filter condition that determines which rows should be updated.

99.What is Self-Join?

A.Self-join is query used to join a table to itself. Aliases should be used for the same table comparison.

100.What is Cross Join?

A.Cross Join will return all records where each row from the first table is combined with each row from the second table.

101.What is a view?

A.Views are virtual tables. Unlike tables that contain data, views simply contain queries that dynamically retrieve data when used.

102.What is a materialized view?

A.Materialized views is also a view but are disk based . Materialized views get updated on specific duration, base upon the interval specified in the query definition. We can index materialized view.