SQL Server interview questions

Database Programming / SQL Server questions 2


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.
For more information see pages from books online titled: “Join Fundamentals” and “Using Joins”.
Can you have a nested transaction?Yes, very much. Check out BEGIN TRAN, COMMIT, ROLLBACK, SAVE TRAN and @@TRANCOUNT
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.
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().
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()
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.
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.
CREATE TABLE emp (empid int,mgrid int,empname char(10))
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’
SELECT t1.empname [Employee], t2.empname [Manager]FROM emp t1, emp t2WHERE t1.mgrid = t2.empid
Here’s an advanced query using a LEFT OUTER JOIN that even returns the employees without managers (super bosses)
SELECT t1.empname [Employee], COALESCE(t2.empname, ‘No manager’) [Manager]FROM emp t1 LEFT OUTER JOINemp t2ON t1.mgrid = t2.empid
Given an employee table, how would you find out the second highest salary?

SQL Server Interview Questions 6

SQL Server Interview Questions
What is normalization? – Well a relational database is basically composed of tables that contain related data. So the Process of organizing this data into tables is actually referred to as normalization.
What is a Stored Procedure? – Its nothing but a set of T-SQL statements combined to perform a single task of several tasks. Its basically like a Macro so when you invoke the Stored procedure, you actually run a set of statements.
Can you give an example of Stored Procedure? – sp_helpdb , sp_who2, sp_renamedb are a set of system defined stored procedures. We can also have user defined stored procedures which can be called in similar way.
What is a trigger? – Triggers are basically used to implement business rules. Triggers is also similar to stored procedures. The difference is that it can be activated when data is added or edited or deleted from a table in a database.
What is a view? – If we have several tables in a db and we want to view only specific columns from specific tables we can go for views. It would also suffice the needs of security some times allowing specfic users to see only specific columns based on the permission that we can configure on the view. Views also reduce the effort that is required for writing queries to access specific columns every time.
What is an Index? – When queries are run against a db, an index on that db basically helps in the way the data is sorted to process the query for faster and data retrievals are much faster when we have an index.
What are the types of indexes available with SQL Server? – There are basically two types of indexes that we use with the SQL Server. Clustered and the Non-Clustered.
What is the basic difference between clustered and a non-clustered index? – The difference is that, Clustered index is unique for any given table and we can have only one clustered index on a table. The leaf level of a clustered index is the actual data and the data is resorted in case of clustered index. Whereas in case of non-clustered index the leaf level is actually a pointer to the data in rows so we can have as many non-clustered indexes as we can on the db.
What are cursors? – Well cursors help us to do an operation on a set of data that we retreive by commands such as Select columns from table. For example : If we have duplicate records in a table we can remove it by declaring a cursor which would check the records during retreival one by one and remove rows which have duplicate values.
When do we use the UPDATE_STATISTICS command? – This command is basically used when we do a large processing of data. If we do a large amount of deletions any modification or Bulk Copy into the tables, we need to basically update the indexes to take these changes into account. UPDATE_STATISTICS updates the indexes on these tables accordingly.
Which TCP/IP port does SQL Server run on? – SQL Server runs on port 1433 but we can also change it for better security.
From where can you change the default port? – From the Network Utility TCP/IP properties –> Port number.both on client and the server.
Can you tell me the difference between DELETE & TRUNCATE commands? – 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.
Can we use Truncate command on a table which is referenced by FOREIGN KEY? – No. We cannot use Truncate command on a table with Foreign Key because of referential integrity.
What is the use of DBCC commands? – DBCC stands for database consistency checker. We use these commands to check the consistency of the databases, i.e., maintenance, validation task and status checks.
Can you give me some DBCC command options?(Database consistency check) – DBCC CHECKDB – Ensures that tables in the db and the indexes are correctly linked.and DBCC CHECKALLOC – To check that all pages in a db are correctly allocated. DBCC SQLPERF – It gives report on current usage of transaction log in percentage. DBCC CHECKFILEGROUP – Checks all tables file group for any damage.
What command do we use to rename a db? – sp_renamedb ‘oldname’ , ‘newname’
Well sometimes sp_reanmedb may not work you know because if some one is using the db it will not accept this command so what do you think you can do in such cases? – In such cases we can first bring to db to single user using sp_dboptions and then we can rename that db and then we can rerun the sp_dboptions command to remove the single user mode.
What is the difference between a HAVING CLAUSE and a WHERE CLAUSE? – Having Clause is basically used only with the GROUP BY function in a query. WHERE Clause is applied to each row before they are part of the GROUP BY function in a query.
What do you mean by COLLATION? – Collation is basically the sort order. There are three types of sort order Dictionary case sensitive, Dictonary – case insensitive and Binary.
What is a Join in SQL Server? – Join actually puts data from two or more tables into a single result set.
Can you explain the types of Joins that we can have with Sql Server? – There are three types of joins: Inner Join, Outer Join, Cross Join
When do you use SQL Profiler? – SQL Profiler utility allows us to basically track connections to the SQL Server and also determine activities such as which SQL Scripts are running, failed jobs etc..
What is a Linked Server? – Linked Servers is a concept in SQL Server by which we can add other SQL Server to a Group and query both the SQL Server dbs using T-SQL Statements.
Can you link only other SQL Servers or any database servers such as Oracle? – We can link any server provided we have the OLE-DB provider from Microsoft to allow a link. For Oracle we have a OLE-DB provider for oracle that microsoft provides to add it as a linked server to the sql server group.
Which stored procedure will you be running to add a linked server? – sp_addlinkedserver, sp_addlinkedsrvlogin
What are the OS services that the SQL Server installation adds? – MS SQL SERVER SERVICE, SQL AGENT SERVICE, DTC (Distribution transac co-ordinator)
Can you explain the role of each service? – SQL SERVER – is for running the databases SQL AGENT – is for automation such as Jobs, DB Maintanance, Backups DTC – Is for linking and connecting to other SQL Servers
How do you troubleshoot SQL Server if its running very slow? – First check the processor and memory usage to see that processor is not above 80% utilization and memory not above 40-45% utilization then check the disk utilization using Performance Monitor, Secondly, use SQL Profiler to check for the users and current SQL activities and jobs running which might be a problem. Third would be to run UPDATE_STATISTICS command to update the indexes
Lets say due to N/W or Security issues client is not able to connect to server or vice versa. How do you troubleshoot? – First I will look to ensure that port settings are proper on server and client Network utility for connections. ODBC is properly configured at client end for connection ——Makepipe & readpipe are utilities to check for connection. Makepipe is run on Server and readpipe on client to check for any connection issues.
What are the authentication modes in SQL Server? – Windows mode and mixed mode (SQL & Windows).
Where do you think the users names and passwords will be stored in sql server? – They get stored in master db in the sysxlogins table.
What is log shipping? Can we do logshipping with SQL Server 7.0 – Logshipping is a new feature of SQL Server 2000. We should have two SQL Server – Enterprise Editions. From Enterprise Manager we can configure the logshipping. In logshipping 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 we can use this as the DR (disaster recovery) plan.
Let us say the SQL Server crashed and you are rebuilding the databases including the master database what procedure to you follow? – For restoring the master db we have to stop the SQL Server first and then from command line we can type SQLSERVER –m which will basically bring it into the maintenance mode after which we can restore the master db.
Let us say master db itself has no backup. Now you have to rebuild the db so what kind of action do you take?
What is BCP? When do we use it? – BulkCopy is a tool used to copy huge amount of data from tables and views. But it won’t copy the structures of the same.
What should we do to copy the tables, schema and views from one SQL Server to another? – We have to write some DTS packages for it.
What are the different types of joins and what dies each do? What are the four main query statements?
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