Sql Subqueries

A subquery is a SQL query or statement or expression within a query. Subqueries are called nested queries or inner queires. It must be enclosed with parenthesis.

Subqueries are provide data or result to the enclosing query. It can return individual values or a list of rows to outer query or main query.

It can be nested into any valid sql statement, expression, or inside another subquery.

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Sql Subquery Syntax

There is no general syntax; subqueries are normal sql queries placed inside parenthesis. Subqueries can be used in many ways and at many locations inside a query or expression:

A subquery usually take one of these below formats:

WHERE expression [NOT] IN (subquery)
WHERE expression comparison_operator [ANY | ALL] (subquery)
WHERE [NOT] EXISTS (subquery)

Sample Database Table - Employee

ID	EmpName	Designation	Dept	JoinYear	Salary
1	Geetha	SQL Database	Oracle	2013	10820
2	Bala Murugan	Cloud Database	Oracle	2015	11660
3	Hanumanthan	Cloud Database	SQL Server	2014	20270.8
4	Ranjani Mai	Sql Head	PHP	2013	8510.8
5	Padmavathi	SQL Mining	SQL Server	2013	3470.4
6	Vinoth Kumar	Database Designer	SQL Server	2014	14600

SQL Subquery With IN Operator

The following SQL statement will display records from employee table which is matched in the IN list.


SELECT * FROM Employee 
WHERE Dept IN (SELECT Dept FROM Employee WHERE Salary < 10000)

In the above query, "SELECT Dept FROM Employee WHERE Salary < 10000" is a subquery and it return list of values to outer query.

The sub-query returns "PHP" and "Sql Server" for which employee "Salary" is less than "10000".

The result of above query is:

ID	EmpName	Designation	Dept	JoinYear	Salary
3	Hanumanthan	Cloud Database	SQL Server	2014	20270.8
4	Ranjani Mai	Sql Head	PHP	2013	8510.8
5	Padmavathi	SQL Mining	SQL Server	2013	3470.4
6	Vinoth Kumar	Database Designer	SQL Server	2014	14600

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