如何检查子查询什么都不返回？

Question

Here is my query:

SELECT
    CASE WHEN if_this_matches_nothing(SELECT 1 FROM mytable WHERE id = :id) THEN 0
    ELSE 1 END

Is there such if_this_matches_nothing function in MySQL ?

Answer 1

You are looking for NOT EXISTS :

SELECT (CASE WHEN NOT EXISTS (SELECT 1 FROM mytable WHERE id = :id)
             THEN 0 ELSE 1
        END) as flag

You can actually shorten this. The CASE is not needed in MySQL:

SELECT (NOT EXISTS (SELECT 1 FROM mytable WHERE id = :id) ) as flag