The query optimizer automatically rewrites as joins many of the queries that make use of subqueries. The conversion is performed without any user action. This section describes which subqueries can be converted to joins so you can understand the performance of queries in your database.

The criteria that must be satisfied in order for a multi-level query to be able to be rewritten with joins differ for the various types of operators, and the structures of the query and of the subquery. Recall that when a subquery appears in the query's WHERE clause, it is of the form

SELECT select-list 
FROM table 
WHERE
[NOT] expression comparison-operator ( subquery-expression ) 
| [NOT] expression comparison-operator { ANY | SOME } ( subquery-expression ) 
| [NOT] expression comparison-operator ALL ( subquery-expression ) 
| [NOT] expression IN ( subquery-expression )
| [NOT] EXISTS ( subquery-expression ) 
GROUP BY group-by-expression 
HAVING search-condition

For example, consider the request, "When did Mrs. Clarke and Suresh place their orders, and by which sales representatives?" It can be answered with the following query:

SELECT OrderDate, SalesRepresentative
FROM SalesOrders
WHERE CustomerID IN (
   SELECT ID
   FROM Customers
   WHERE Surname = 'Clarke' OR GivenName = 'Suresh' );

The subquery yields a list of customer IDs that correspond to the two customers whose names are listed in the WHERE clause, and the main query finds the order dates and sales representatives corresponding to those two people's orders.

The same question can be answered using joins. Here is an alternative form of the query, using a two-table join:

SELECT OrderDate, SalesRepresentative
FROM SalesOrders, Customers
WHERE CustomerID=Customers.ID AND
  ( Surname = 'Clarke' OR GivenName = 'Suresh' );

This form of the query joins the SalesOrders table to the Customers table to find the orders for each customer, and then returns only those records for Suresh and Clarke.

 Case where a subquery works, but a join does not

SELECT Name, Description, Quantity
FROM Products
WHERE Quantity <  2 * (
   SELECT AVG( Quantity )
   FROM SalesOrderItems );

 See also