SQL - Sub Query

In SQL Subquery is a query within another SQL query and embedded within the WHERE clause. It is used to return data which is used in the main query as a condition to further restrict the data to be retrieved.

A Subquery can be used with SELECT, UPDATE, INSERT, DELETE statements along with expression operator. It could be equality operator or comparison operator such as =, >, =, <= and Like operator.

There are a few rules that a subquery must follow:

• A subquery must be enclosed within parentheses.
• A subquery can be placed in a number of SQL clauses: WHERE clause, HAVING clause, FROM clause.
• A subquery is a query within another query. The outer query is known as the main query, and the inner query is known as a subquery.
• The subquery generally executes first, and its output is used to complete the query condition for the main or outer query.
• An ORDER BY command cannot be used in a subquery, although the main query can use an ORDER BY. The GROUP BY command can be used to perform the same function as the ORDER BY in a subquery.
• Subqueries are on the right side of the comparison operator.
• A subquery can have only one column in the SELECT clause, unless multiple columns are in the main query for the subquery to compare its selected columns.
• A subquery which returns more than one row can only be used with multiple value operators such as the IN operator.
• The BETWEEN operator cannot be used with a subquery. However, the BETWEEN operator can be used within the subquery.

Subquery with the SELECT Statement

SQL subqueries are most frequently used with the SELECT statement.

Syntax

SELECT column_name  
FROM table_name  
WHERE column_name expression operator   
( SELECT column_name  from table_name WHERE ... );  

Example:

Consider a database table called Employee with the following records:

A subquery with SELECT statement will be:

SELECT * FROM Employee
  WHERE EmpID IN (SELECT EmpID   
    FROM Employee   
    WHERE Salary > 2800);

This will produce the result as shown below:

Subquery with the INSERT Statement

A SQL subquery can also be used with the INSERT statement. In the insert statement, data returned from the subquery is used to insert into another table. The selected data in the subquery can be modified with any of the character, date or number functions.

Syntax

INSERT INTO table_name (column1, column2, column3....)   
SELECT *  
FROM table_name  
WHERE VALUE OPERATOR  

Example:

Consider a table Employee_Temp with similar structure as Employee table. To copy the complete Employee table into the Employee_Temp table, the following query can be used:

INSERT INTO Employee_Temp  
   SELECT * FROM Employee   
   WHERE EmpID IN (SELECT EmpID   
   FROM Employee);  

Subquery with the UPDATE Statement

A SQL subquery can also be used in conjunction with the UPDATE statement. Either single or multiple columns in a table can be updated when using a subquery with the UPDATE statement.

Syntax

UPDATE table  
SET column_name = new_value  
WHERE VALUE OPERATOR  
   (SELECT COLUMN_NAME  
   FROM TABLE_NAME  
   WHERE condition);  

Example:

Consider a database table called Employee with the following records:

A subquery with UPDATE statement will be:

UPDATE Employee  
  SET Salary = Salary * 1.2  
  WHERE Salary IN (SELECT Salary FROM Employee  
    WHERE Salary <= 2800);  

--see the result
SELECT * FROM Employee;

This will produce the result as shown below:

Subquery with the DELETE Statement

A SQL subquery can also be used in conjunction with the DELETE statement like with any other statements mentioned above.

Syntax

DELETE FROM TABLE_NAME  
WHERE VALUE OPERATOR  
   (SELECT COLUMN_NAME  
   FROM TABLE_NAME  
   WHERE condition);  

Example:

Consider a database table called Employee with the following records:

A subquery with UPDATE statement will be:

DELETE FROM Employee  
  WHERE Age IN (SELECT Age FROM Employee  
  WHERE Age >= 28 );  

--see the result
SELECT * FROM Employee;

This will produce the result as shown below: