What is subquery in SQL?

In SQL a Subquery can be simply defined as a query within another query. In other words we can say that a Subquery is a query that is embedded in WHERE clause of another SQL query.

Important rules for Subqueries:

• 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.
• 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.
• Subquery must be enclosed in parentheses.
• Subqueries are on the right side of the comparison operator.
• ORDER BY command cannot be used in a Subquery. GROUPBY command can be used to perform same function as ORDER BY command.
• Use single-row operators with singlerow Subqueries. Use multiple-row operators with multiple-row Subqueries.