如何检查子查询返回什么？

Question

这里是我的查询：

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

有这样if_this_matches_nothing功能在MySQL？

Answer 1

您正在寻找NOT EXISTS：

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

实际上，你可以缩短这个。在MySQL中不需要CASE：

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