MySQL的拿幾張列關聯在一個查詢

值我有2個表：MySQL的拿幾張列關聯在一個查詢

STEP 
id 
value 




SCALE 
id 
s1 
s2 
s3 
s4 
s5 
s6

現在step.id是外鍵scale.s1，scale.s2，scale.s3等我想規模.s1-s6值來自步驟。我已經用嵌套選擇書寫了這個查詢。有一個選擇可以得到這個值嗎？

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 ;

來源

2013-02-08 Stathis G.

只需加入STEP六次。 – 2013-02-08 20:22:04

我會考慮設計不同的表格。 Step {StepId，value}，Scale {ScaleId}，ScaleStep {ScaleId，StepId}，以便您可以進行簡單的連接。 – 2013-02-08 20:48:38

你可以給我加入步驟六次的語法請@ C.Ramseyer因爲我有麻煩得到它... thnx – 2013-02-08 23:19:12

只需加入多次。以下是前三項：

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

來源

2013-02-08 23:39:29

您可以使用單個查詢與MAX和CASE做到這一點：

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

這裏是Fiddle。

祝你好運。

來源

2013-02-08 20:41:10 sgeddes

MySQL的拿幾張列關聯在一個查詢

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