MySQL查询与基于其他操作结果的操作

Question

我需要根据其他操作结果进行操作。例如：

SELECT
    SUM(first_column) as res1,
    SUM(second_column) as res2,
    res1/res2 as res3 
FROM my_table

但似乎只能对现有列进行操作。这个用例的解决方法是什么？

Answer 1

您可以使用子查询来实现这一目标。 像这样：

SELECT res1, res2, res1/res2 as res3 
(SELECT SUM(xxx) as res1, SUM(xxx) as res2
FROM yyy) as subquery

Answer 2

尝试类似：

SELECT res1/res2 as res3
FROM (SELECT SUM(xxx) as res1, SUM(xxx) as res2 
      FROM yyy))

Answer 3

SELECT res1/res2 as res3
FROM (SELECT SUM(xxx) as res1, SUM(xxx) as res2 
FROM yyy))

这应该有效

Answer 4

您可以使用此代码块解决您的问题：

select t.res1
  ,t.res2
  ,t.res1 / t.res2 as res3
from (select sum(xxx) as res1
          ,sum(xxx) as res2
      from yyy) t

或

不推荐，但有效。

select sum(xxx1) as res1
  ,sum(xxx2) as res2
  ,sum(xxx1) / sum(xxx2) as res3
  from yyy

Answer 5

如果您不需要中间结果，则可以通过跳过子查询来获得更好的性能：

SELECT SUM(c1) / SUM(c2) AS res FROM t