Sql从结果列中选择最大值

Question

我想从另一个查询结果中选择MAX，它返回的值只有一列。

这给我列了所有从中我想，只选择最高值的查询是：

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

这将返回

PERF 
-2.9 
1.4 
12.5 
14 

现在我要选择的最大值从那。我想是这样..

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

Answer 1

使用你的第一个查询作为子查询，但在一个字段中使用MAX，而不是在查询

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，你也可以做

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

，或者如果不是有

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

编辑

从你的评论，你可以做

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