As you have seen, subqueries usually appear in the HAVING clause or the WHERE clause of a query. A subquery may itself contain a WHERE clause and/or a HAVING clause, and, consequently, a subquery may appear in another subquery. Subqueries inside other subqueries are called nested subqueries.
List the order IDs and line IDs of those orders shipped on the same day when any item in the fees department was ordered.
SELECT ID, LineID FROM SalesOrderItems WHERE ShipDate = ANY ( SELECT OrderDate FROM SalesOrders WHERE FinancialCode IN ( SELECT Code FROM FinancialCodes WHERE ( Description = 'Fees' ) ) );
In this example, the innermost subquery produces a column of financial codes whose descriptions are "Fees":
SELECT Code FROM FinancialCodes WHERE ( Description = 'Fees' );
The next subquery finds the order dates of the items whose codes match one of the codes selected in the innermost subquery:
SELECT OrderDate FROM SalesOrders WHERE FinancialCode IN ( subquery-expression );
Finally, the outermost query finds the order IDs and line IDs of the orders shipped on one of the dates found in the subquery.
SELECT ID, LineID FROM SalesOrderItems WHERE ShipDate = ANY ( subquery-expression );
Nested subqueries can also have more than three levels. Though there is no maximum number of levels, queries with three or more levels take considerably longer to run than do smaller queries.