What is subquery?

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Answer:

A subquery is a query within another query. The outer query is called as main query, and inner query is called subquery. SubQuery is always executed first, and the result of subquery is passed on to the main query.

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832. What are the types of subquery?

Answer:

There are two types of subquery – Correlated and Non-Correlated.

A correlated subquery cannot be considered as independent query, but it can refer the column in a table listed in the FROM the list of the main query.

A Non-Correlated sub query can be considered as independent query and the output of subquery are substituted in the main query.

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833. What is a stored procedure?

Answer: A stored procedure is a pre-compiled set of one or more SQL statements that are stored on SQL Server. The benefit of Stored Procedures is that they are executed on the server-side and perform a set of actions, before returning the results to the client-side. This allows a set of actions to be executed with minimum time and also reduces the network traffic. Hence stored procedure improves performance to execute SQL statements.

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843. When to use single-column index or a composite index?

Answer:

To create a single-column index or a composite index, take into consideration the column(s) that you may use very frequently in a query's WHERE clause as filter conditions.

Should there be only one column used, a single-column index should be the choice. Should there be two or more columns that are frequently used in the WHERE clause as filters, the composite index would be the best choice.

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Answer:

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What is subquery?

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832. What are the types of subquery?

833. What is a stored procedure?

841. When Right Join gives result like Left Join?

842. When Left Join gives result like Right Join?

843. When to use single-column index or a composite index?

844. When should indexes be avoided?

845. When should indexes be created?

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882. JOIN or INNER JOIN in SQL Server

883. LEFT JOIN or LEFT OUTER JOIN in SQL Server