Sql从结果列中选择max

Question

我试图从另一个查询结果中选择MAX，该结果只返回一个带有值的列。

这个查询为我提供了我想要只选择最大值的所有值的列：

SELECT perfy as perf
FROM aasv_chart5
UNION
SELECT perfcumu as perf
FROM aasv_chart5

返回

PERF
-2.9
1.4
12.5
14

现在我想从中选择最大值。我试过像..

select MAX(SELECT perfy as perf
    FROM aasv_chart5
    UNION
    SELECT perfcumu as perf
    FROM aasv_chart5) FROM aasv_chart5

Answer 1

将您的第一个查询用作子查询，但在字段上使用MAX，而不是在查询上使用

SELECT MAX(perf) from 
  (SELECT perfy as perf
   FROM aasv_chart5
   UNION
   SELECT perfcumu as perf
   FROM aasv_chart5) s

取决于您的dbms，您也可以

select max(greatest(perfy, perdcumu))
from aasv_chart5

或greatest不在那里

select max(case when perfy > perdcumu then perfy else perdcumu end)
from aasv_chart5

修改

从你的评论

，你可以做到

select id, 
  (select max(greatest(perfy, perdcumu))
  from aasv_chart5) as maxVal
from aasv_chart5

或

select id, s.maxval
from aasv_chart5
cross join  (select max(greatest(perfy, perdcumu)) as maxval
  from aasv_chart5) s

Answer 2

SELECT max(abc.perf)
FROM
(SELECT perfy as perf
    FROM aasv_chart5
    UNION
    SELECT perfcumu as perf
    FROM aasv_chart5) as abc