SQL Database Concepts Interview Preparation Guide
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SQL Database Concepts frequently Asked Questions by expert members with experience in Basic SQL Database Concepts. These interview questions and answers on SQL Database Concepts will help you strengthen your technical skills, prepare for the interviews and quickly revise the concepts. So get preparation for the SQL Database Concepts job interview

113 Basic SQL Server Questions and Answers:

1 :: Explain Differentiate between a HAVING CLAUSE and a WHERE CLAUSE?

HAVING CLAUSE

- HAVING CLAUSE is used only with the SELECT statement.
- It is generally used in a GROUP BY clause in a query.
- If GROUP BY is not used, HAVING works like a WHERE clause.

WHERE Clause

- It is applied to each row before they become a part of the GROUP BY function in a query.
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2 :: Do you know query execution plan?

- The optimizer available in SQL Server optimizes the code to be effectively executed.
- A query execution plan shows how this optimizer would run the query.
- Query execution plan can be viewed by :
- Using the Show Execution Plan option available in Query Analyzer,
- Displaying Estimated Execution Plan on the query dropdown menu,
- Use the SET SHOWPLAN_TEXT ON command before running a query and capturing the execution plan event in a SQL Server Profiler trace.
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3 :: Explain Comment on Transactions?

- Using transactions we can group all SQL commands into a single unit.
- The transaction begins with some task and finishes only when all tasks within it are over.
- The transaction gets over successfully only when all commands in it are successfully over. Even if one command fails, the whole transaction fails.
- The BEGIN TRANSACTION, ROLLBACK TRANSACTION, and COMMIT TRANSACTION statements are used to work with transactions.
- A group of tasks starts with the begin statement.
- In case of any problem, the rollback command is executed to abort the transaction.
- If all the tasks run successfully, all commands are executed through commit statement.
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4 :: Do you know what is recursion? Is it possible for a stored procedure to call itself or recursive stored procedure? How many levels of SP nesting is possible?

Recursion is method of problem solving where the solution is arrived at by repetitively applying the logic and solution to the subsets of the problem.

Transact-SQL supports recursion. So, yes it is possible for a stored procedure to call itself.

Stored procedures and managed code references can be nested up to 32 levels.
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5 :: Tell me what do you mean by an execution plan? Why is it used? How would you view it?

a.) An execution plan can be called as a road map that graphically or textually shows the data retrieval methods which have been chosen by the SQL
Server query optimizer, for a stored procedure or ad- hoc query.

b.) It is used because it is a very useful tool for a developer to understand the performance characteristics of a query or stored procedure.

c.) There exists an option called "Show Execution Plan" in Query Analyzer. If this option is turned on, it will display query execution plan in separate window when the query is run again.
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6 :: Tell me When is the UPDATE_STATISTICS command used?

- When the processing of large data is done, this command is used.
- Whenever large number of deletions, modification or copy takes place into the tables, the indexes need to be updated to take care of these changes. UPDATE_STATISTICS performs this job.
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7 :: Tell me what do you understand by a view? What does the WITH CHECK OPTION clause for a view do?

- A view is a virtual table that consists of fields from one or more real tables.
- It is usually used to join multiple tables and get the data.
- The WITH CHECK OPTION for a view prevents any modification to the data that does not confirm to the WHERE clause of the view definition.
- This allows the data belonging to the view to be updated through the view.
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8 :: Explain what is the function of SQL Server Agent Windows service?

- It is a Windows service which handles the tasks scheduled within the SQL Server environment. These tasks are also called as job and are stored with in SQL server. The jobs may run through a trigger, a predefined schedule or on demand.
- This service is very useful in determining why a particular job did not run as intended.
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9 :: Can you please differentiate between a primary key and a unique key?

- By default, clustered index on the column are created by the primary key whereas nonclustered index are created by unique key.
- Primary key doesn't allow NULLs, but unique key allows one NULL.
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10 :: Tell me what are the advantages of using Stored Procedures?

- They help in reducing the network traffic and latency which in turn boosts application performance.
- They help in promoting code reuse.
- They provide better security to data.
- It is possible to encapsulate the logic using stored procedures. This allows to change stored procedure code without affecting clients.
- It is possible to reuse stored procedure execution plans, which are cached in SQL Server's memory. This reduces server overhead.
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