根據 MySQL 中的值選擇字段

Question

有 mysql 表foo ：

a | b | c
----------
1 | 2 | 0
2 | 3 | 1
3 | 4 | 0

和一個查詢

SELECT a-b as subA, b-a as subB FROM foo;

subA | subB
-----------
 -1  |   1
 -1  |   1
 -1  |   1

如果 c = 0，我如何選擇ab as sum ba as sum如果 c = 1，我如何選擇ba as sum ，以便得到以下結果：

sum
---
-1
 1
-1

Answer 1

SELECT (a-b)*(1-2*c) AS `sum`
FROM foo

Answer 2

您可以使用case表達式：

select f.*,
    case c 
        when 0 then a - b 
        when 1 then b - a
    end as res
from foo f

如果c總是0或1 ，我們可以對sign()有點幻想：

select f.*, sign(c - 0.5) * (b - a) as res 
from foo f

Answer 3

這非常簡單（查看SQLize.online ）：

SELECT 
    a-b as subA, 
    b-a as subB,
    CASE 
        WHEN c = 0 THEN a-b
        WHEN c = 1 THEN b-a
    END as sum
FROM foo;

結果：

+======+======+=====+
| subA | subB | sum |
+======+======+=====+
| -1   | 1    | -1  |
+------+------+-----+
| -1   | 1    | 1   |
+------+------+-----+
| -1   | 1    | -1  |
+------+------+-----+