IN Operator

The IN operator tests set membership.It means "equal to any member of." The set can contain nulls, butthey are ignored. For example, the following expression testswhether a value is part of a set of values:

letter IN ('a','b','c')

Be careful when inverting this condition. Expressions ofthe form:

value NOT IN set

yield FALSE if the set contains anull.--如果這集合（set）包括了空值，那麼這個表示式將產生FALSE。 例如：

select dest,revenue from test where revenue not in(
select 4000 from test union select null fromdual);--此結果將返回0行。

對於子查詢：select 4000 from test union select null from dual來說，這個結果集有NULL值。

而且使用運算子NOT IN。因此WHERE 條件為FALSE。因此不會返回的行。