Select 字段基于 MySQL 中的值

Question

有mysqltablefoo:

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

和一个查询

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

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

我如何select a-b as sum 如果 c = 0 和 b-a as sum 如果 c = 1，以便我得到这个结果：

sum
---
-1
 1
-1

Answer 1

您可以使用 case 表达式：

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

如果 c 总是 0 或 1，我们可以用 sign():

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

Answer 2

这很简单（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;

结果：

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

Answer 3

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

Select 字段基于 MySQL 中的值

Select field based on value in MySQL

mysql

sql

case