Subquery Syntax

SELECT select_list

FROM   table

WHERE  expr operator

                    (SELECT select_list

                     FROM table);

• The subquery(inner query) executes once before the main query(outer query).
• The result of the subquery is used by the main query.
  

You can place the subquery in a number of SQL clauses, including the following:

• WHERE clause
• HAVING clause
• FROM clause