SQL Subqueries

• A subquery cannot be immediately enclosed in a set function.
• You can place the Subquery in a number of SQL clauses: WHERE clause, HAVING clause, FROM clause.
  Subqueries 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.
• Subquery must be enclosed in parentheses.
• A subquery is a query within another query. The outer query is called as main query and inner query is called as 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 can return individual values or a list of records.

Syntax:
There is not any general syntax for Subqueries are regular queries placed inside parenthesis.

Subqueries are seen to be used most frequently with SELECT statement as shown below:

Here is an subquery with the IN operator.

Subqueries can also assign column values for each record:

SQL Subquery Examples

Problem: List products with order quantities greater than 100.