MySQL选择查询

我有三张桌子t1、t2和t3。下面是他们的模式

我需要加入他们，并且有条件

In t1 table, objectName can be one of 4: 'ABC', 'DEF', 'GHI',  or 'JKL'.
If t1.objectName is 'ABC', I need to join t1.idObject with t2.id1. 
If t1.objectName is 'DEF', I need to join t1.idObject with with t2.id2. 
If t1.objectName is 'GHI', I need to join t1.idObject with t2.id3. 
If t1.objectName is 'JKL', I need to join t1.idObject with t3.id4.

I need to select t1.idObject, t1.objectName, t2.custName, t4.Message.

另外，如果t1.objectName是'DEF'或'GHI'，我希望custName在select结果中为null

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如有疑问或解释，将不胜感激

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非常感谢。

SQL背后的思想是，它允许您用SQL术语表达确切的需求，例如：

SELECT
 whatever you need, including
 CASE WHEN t1.objectName in ( 'DEF' , 'GHI') 
  THEN NULL
  ELSE t2.custName
 END AS custName
FROM
 t1, t2, t3
WHERE
 (t1.objectName = 'ABC' AND t1.idObject = t2.id1) 
 OR
 (t1.objectName = 'DEF' AND t1.idObject = t2.id2)
 OR
 ...

---

我希望您可以将其扩展到所有需求。

使用联合来组合每个查询的结果：

SELECT t1.idObject, t1.objectName, t2.custName, NULL Message
FROM t1 JOIN t2 ON (t1.idObject = t2.id1)
WHERE t1.objectName = "ABC"
UNION ALL
SELECT t1.idObject, t1.objectName, NULL custName, NULL Message
FROM t1 JOIN t2 ON (t1.idObject = t2.id2)
WHERE t1.objectName = "DEF"
UNION ALL
SELECT t1.idObject, t1.objectName, NULL custName, NULL Message
FROM t1 JOIN t2 ON (t1.idObject = t2.id3)
WHERE t1.objectName = "GHI"
UNION ALL
SELECT t1.idObject, t1.objectName, NULL custName, Message
FROM t1 join t3 ON (t1.idObject = t3.id4)
WHERE t1.objectName = "JKL"

您没有提到它，但我假设在ABC、DEF、GHI案例中，

Message

应该为空，因为在

t2

中没有这样的列，而在JKL案例中，

custName

应该为空，因为在

t3

中没有这样的列

要对结果进行排序，需要将UNION设置为子查询：

SELECT * FROM (
    SELECT t1.idObject, t1.objectName, t2.custName, NULL Message
    FROM t1 JOIN t2 ON (t1.idObject = t2.id1)
    WHERE t1.objectName = "ABC"
    UNION ALL
    SELECT t1.idObject, t1.objectName, t2.custName, NULL Message
    FROM t1 JOIN t2 ON (t1.idObject = t2.id2)
    WHERE t1.objectName = "DEF"
    UNION ALL
    SELECT t1.idObject, t1.objectName, t2.custName, NULL Message
    FROM t1 JOIN t2 ON (t1.idObject = t2.id3)
    WHERE t1.objectName = "GHI"
    UNION ALL
    SELECT t1.idObject, t1.objectName, NULL custName, Message
    FROM t1 join t3 ON (t1.idObject = t3.id4)
    WHERE t1.objectName = "JKL" ) x
ORDER BY idObject, custName, Message

---

您可以使用大小写语法：您想选择

t4.Message

？没有表

t4

，所以我猜你的意思是

t3

。但是当

objectName

是

JKL

时，您只能使用

t3

进行连接，否则应该是什么呢？任何包含三个不同标识列的表都代表着糟糕的设计。我认为这是一个家庭作业，考虑到设计强制大量使用连接和条件，并且这里似乎没有上下文。也许我想错了。哇！你真的试过这个吗？假设

t1

和

t2

成功连接，那么

t3

中的每一行现在都将连接到该结果中。。。如果你尝试这个，你会看到你得到了疯狂的结果。没错。如果问题只是决定要与

t2

中的哪一列联接，那么这就行了，但是由于要联接的表也依赖于

objectName

，因此它根本不能正常工作。在上述要求中，如果我删除“如果t1.objectName是'DEF'或'GHI'，我希望custName在select结果中为null。”查询是什么样子的？你能在下面发帖吗？

表现很差

很大方，另一个答案给出了完全愚蠢的结果。然而，这一个从我这里得到+1:）在上述要求中，如果我删除“如果t1.objectName是'DEF'或'GHI'，我希望custName在select结果中为null。”查询会是什么样子？请您在下面发帖好吗？只需将

t2.custName

放在那些SELECT子句中，而不是

NULL custName

@Barmar如何在上面的查询中按订单？只是为了使用单一订单。。。？按idObject、objectName、custName、Message排序。谢谢

SELECT * FROM (
    SELECT t1.idObject, t1.objectName, t2.custName, NULL Message
    FROM t1 JOIN t2 ON (t1.idObject = t2.id1)
    WHERE t1.objectName = "ABC"
    UNION ALL
    SELECT t1.idObject, t1.objectName, t2.custName, NULL Message
    FROM t1 JOIN t2 ON (t1.idObject = t2.id2)
    WHERE t1.objectName = "DEF"
    UNION ALL
    SELECT t1.idObject, t1.objectName, t2.custName, NULL Message
    FROM t1 JOIN t2 ON (t1.idObject = t2.id3)
    WHERE t1.objectName = "GHI"
    UNION ALL
    SELECT t1.idObject, t1.objectName, NULL custName, Message
    FROM t1 join t3 ON (t1.idObject = t3.id4)
    WHERE t1.objectName = "JKL" ) x
ORDER BY idObject, custName, Message