You can use the ALL keyword to determine whether a value is less than or greater than all the values in a subquery result. The important characteristics of subquery comparisons that use ALL are:
ALL modifies a comparison operator in a subquery comparison test and follows =, <>, <, <=, >, or >=; see “Comparing a Subquery Value by Using a Comparison Operator” earlier in this chapter.
The combination of a comparison operator and ALL tells the DBMS how to apply the comparison test to the values returned by a subquery. < ALL, for example, means less than every value in the subquery result, and > ALL means greater than every value in the subquery result.
When ALL is used with <, <=, >, or >=, the comparison is ...