即使使用EXISTS也没有结果

Question

I don't undersand why MySQL returns "MySQL did not produce a record" whereas I use EXISTS (I willingly chose a subquery which doesn't produce records) :

SELECT  page_ID
    FROM ranks_update
    WHERE EXISTS (
    SELECT *
    FROM ranks_update
    WHERE ranking_ID = 3
        AND current_rank = 1
        AND rating_time < '2012-08-05 02:57:59'
        AND rating_time >= '2012-08-05 00:00:00'  
    GROUP BY page_ID
);

By definition EXISTS allows to get a result from a query which doesn't return any records. Until now I've always got NULL in such case.

Answer 1

It is returning that message because no records match. NULL is a value for a column. It is quite different from an empty return set.

If you have an aggregate function, then the empty set returns a NULL. So the following would return a NULL value:

select max(Page_ID)

and this would return 0:

select count(Page_ID)