如何在mysql的同一查询中使用手动创建的字段？

Question

这一定是一件非常容易的事情,但我无法完成任务,也无法在任何地方找到这样的问题.这就是我的select代码:

SELECT *,
    (SELECT SUM(x) FROM a WHERE (...)) AS sum_x,
    (SELECT SUM(y) FROM b WHERE (...)) AS sum_y
FROM table

我想创建一个更多的领域(sum_x - sum_y),但当我尝试做这样的事情

SELECT *,
    (SELECT SUM(x) FROM a WHERE (...)) AS sum_x,
    (SELECT SUM(y) FROM b WHERE (...)) AS sum_y,
    (sum_x - sum_y) AS sum_c
FROM table

它对我生气并说Unknown column 'sum_x' in 'field list'.

我在这里失踪了什么？

Answer 1

SQL通常不允许您在定义它们的同一级别引用列别名.您需要使用子查询执行此操作:

select t.*, (sum_x - sum_y) as sum_c
from (SELECT *,
             (SELECT SUM(x) FROM a WHERE (...)) AS sum_x,
             (SELECT SUM(y) FROM b WHERE (...)) AS sum_y
      FROM table
     ) t;

但是,MySQL倾向于实现子查询,因此存在性能损失.实现的开销可能比两次执行子查询计算要少:

      SELECT *,
             (SELECT SUM(x) FROM a WHERE (...)) AS sum_x,
             (SELECT SUM(y) FROM b WHERE (...)) AS sum_y,
             ((SELECT SUM(x) FROM a WHERE (...)) -
              (SELECT SUM(y) FROM b WHERE (...))
             ) as sum_c
      FROM table;

仅进行一次计算也更容易维护和理解.

在MySQL中,您也可以这样做:

      SELECT *,
             @sum_x := (SELECT SUM(x) FROM a WHERE (...)) AS sum_x,
             @sum_y := (SELECT SUM(y) FROM b WHERE (...)) AS sum_y,
             (@sum_x - @sum_y) as sum_c
      FROM table cross join
           (select @sum_x := 0, @sum_y := 0) const

MySQL不保证在赋值后评估最后一个表达式.它似乎在实践中始终这样做.