Omitting the ANY, ALL, or SOME Keywords

You can omit the keywords ANY, ALL, or SOME in a subquery if you know that the subquery will return exactly one value. If you omit the ANY, ALL, or SOME keywords, and the subquery returns more than one value, you receive an error. The subquery in the following example returns only one row because it uses an aggregate function:

SELECT order_num FROM items 
   WHERE stock_num = 9 AND quantity =
      (SELECT MAX(quantity) FROM items WHERE stock_num = 9);