MySQL在一个查询中获取多个列关联值答案

【问题标题】：MySQL get several column associated values in one queryMySQL在一个查询中获取多个列关联值
【发布时间】：2013-02-08 20:10:12
【问题描述】：

我有 2 张桌子：

STEP
id 
value




SCALE
id 
s1
s2
s3
s4
s5
s6

现在 step.id 是 scale.s1、scale.s2、scale.s3 等的外键我想从步骤中获取 scale.s1-s6 值。我用嵌套选择编写了这个查询。是否可以通过一次选择获得此值？

SELECT (
    SELECT step.val FROM step, scale WHERE step.id = scale.s1 AND scale.id = 1) as v1,(
    SELECT step.val FROM step,scale WHERE step.id = scale.s2 AND scale.id = 1) as v2, (
    SELECT step.val FROM step,scale WHERE step.id = scale.s3 AND scale.id = 1) as v3, (
    SELECT step.val FROM step,scale WHERE step.id = scale.s4 AND scale.id = 1) as v4, (
    SELECT step.val FROM step,scale WHERE step.id = scale.s5 AND scale.id = 1) as v5, (
    SELECT step.val FROM step,scale WHERE step.id = scale.s4 AND scale.id = 1) as v6, (
    SELECT step.val FROM step,scale WHERE step.id = scale.s5 AND scale.id = 1) as v7 ;

【问题讨论】：

只要加入STEP六次。
我会考虑以不同的方式设计表格。 Step{StepId, value}, Scale{ScaleId}, ScaleStep{ScaleId, StepId} 所以你可以做一个简单的连接。
你能给我六次加入步骤的语法吗@C.Ramseyer，因为我很难得到它...thnx

标签： mysql select field

【解决方案1】：

只需多次加入步骤即可。这是前三个：

select s.id, s1.val as v1, s2.val as v2, s3.val as v3
from scale s
join step s1 on s.s1 = s1.id
join step s2 on s.s2 = s2.id
join step s3 on s.s3 = s3.id

【讨论】：

【解决方案2】：

您可以使用带有MAX 和CASE 的单个查询来完成：

SELECT
  MAX(CASE WHEN scale.s1 = step.id THEN step.val END) v1,
  MAX(CASE WHEN scale.s2 = step.id THEN step.val END) v2,
  MAX(CASE WHEN scale.s3 = step.id THEN step.val END) v3,
  MAX(CASE WHEN scale.s4 = step.id THEN step.val END) v4,
  MAX(CASE WHEN scale.s5 = step.id THEN step.val END) v5,
  MAX(CASE WHEN scale.s6 = step.id THEN step.val END) v6
FROM step, scale
WHERE scale.id = 1

这里是Fiddle。

祝你好运。

【讨论】：