mysql - 将mysql值分配给变量内联

Question

为什么这不起作用我正在尝试获取以前和当前的值来计算百分比变化。我正确地得到了两个值，但现在我如何重用它们来进行数学运算

当我尝试下面的命令时，我得到错误 1054 (42S22):“字段列表”中的未知列“currentVal”

            SELECT IFNULL(DValue,0) as currentVal, 
                      (SELECT IFNULL(DValue,0) 
                       FROM ...
                       WHERE...) as previousVal, 
                      (currentVal-previousVal)/previousVal
            FROM ...
            WHERE ...;

Answer 1

不能在同一个 SELECT 中引用别名列，必须将其放在子查询中:

SELECT currentVal, previousVal, (currentVal-previousVal)/previousVal
FROM (
            SELECT    IFNULL(DValue,0) as currentVal, 
                      (SELECT IFNULL(DValue,0) 
                       FROM ...
                       WHERE...) as previousVal, 
            FROM ...
            WHERE ...) T;