Sql 有助于更好地理解连接

我有3个表，从中减去数据，为了得到大部分数据，我有一个查询，效果很好，但我不能得到一个特定的行，这就是我需要一些帮助的地方

表1：

EquipmentID  | EquipmentName | EquipmentTypeID
15           | Tesla         | 68
16           | Colombus      | 93

表2：

EquipmentTypeID | DisplayName      | 
68              | Electrical Device|
93              | GPS Device       |

表3：

EquipmentID  | IPAddress  |
15           | 192.168.1.1|  
16           | 192.168.0.1|

到目前为止，我使用以下SQL语法获得的数据如下：

SELECT DISTINCT t1.IPAddress,
t2.EquipmentID
FROM Table3 t1
JOIN Table1 t2 ON t1.EquipmentID = t2.EquipmentID
WHERE IPAddress LIKE '%192%'

我得到的结果看起来像

IPAddress  | EquipmentID |
192.168.1.1| 15          |
192.168.0.1| 16          |

然而，当我进行如下连接时，结果就是一团糟

SELECT DISTINCT t1.IPAddress,
t2.EquipmentID,
t3.EquipmentTypeID,
t4.DisplayName
FROM Table3 t1
JOIN Table1 t2 ON t2.EquipmentID = t1.EquipmentID
JOIN Table2 t3 ON t3.EquipmentTypeID = t1.EquipmentTypeID
JOIN Table2 t4 ON t3.EquipmentTypeID = t1.EquipmentTypeID
WHERE IPAddress LIKE '%192'

但现在我得到的结果是：

IPAddress  | EquipmentID |EquipmentTypeID| DisplayName     |
192.168.1.1| 15          |68             | ElectricalDevice|
192.168.1.1| 15          |93             | GPS Device      |
192.168.0.1| 16          |68             | ElectricalDevice|
192.168.0.1| 16          |93             | GPS Device      |

关于如何为相应的IPAddress和EquipmentID获取正确的显示名称，您有什么想法吗

---

如果你需要更多的澄清，请让我知道。感谢您提前提供的帮助

您两次使用错误的id加入表2：

JOIN Table2 t3 ON t3.EquipmentTypeID = t1.EquipmentTypeID
JOIN Table2 t4 ON t3.EquipmentTypeID = t1.EquipmentTypeID

试着这样做：

SELECT DISTINCT t3.IPAddress,
t2.EquipmentID,
t3.EquipmentTypeID,
t2.DisplayName
FROM Table3 t3
JOIN Table1 t1 ON t1.EquipmentID = t3.EquipmentID
JOIN Table2 t2 ON t2.EquipmentTypeID = t3.EquipmentTypeID
WHERE IPAddress LIKE '%192'

---

注意：我更改了已联接表的名称，因为这有点混乱。

您正在联接表2两次，并且使用了错误的id:

JOIN Table2 t3 ON t3.EquipmentTypeID = t1.EquipmentTypeID
JOIN Table2 t4 ON t3.EquipmentTypeID = t1.EquipmentTypeID

SELECT 
t3.IPAddress,
t3.EquipmentID,
t2.EquipmentTypeID,
t2.DisplayName
FROM Table3 t3
JOIN Table1 t1 ON t3.EquipmentID = t1.EquipmentID
JOIN Table2 t2 ON t2.EquipmentTypeID = t1.EquipmentTypeID
WHERE IPAddress LIKE '%192.%'
GROUP BY
t3.IPAddress,
t3.EquipmentID,
t2.EquipmentTypeID,
t2.DisplayName

试着这样做：

SELECT DISTINCT t3.IPAddress,
t2.EquipmentID,
t3.EquipmentTypeID,
t2.DisplayName
FROM Table3 t3
JOIN Table1 t1 ON t1.EquipmentID = t3.EquipmentID
JOIN Table2 t2 ON t2.EquipmentTypeID = t3.EquipmentTypeID
WHERE IPAddress LIKE '%192'

---

注意：我更改了已联接表的名称，因为这有点混乱。

您正在联接表2两次，并且使用了错误的id:

JOIN Table2 t3 ON t3.EquipmentTypeID = t1.EquipmentTypeID
JOIN Table2 t4 ON t3.EquipmentTypeID = t1.EquipmentTypeID

SELECT 
t3.IPAddress,
t3.EquipmentID,
t2.EquipmentTypeID,
t2.DisplayName
FROM Table3 t3
JOIN Table1 t1 ON t3.EquipmentID = t1.EquipmentID
JOIN Table2 t2 ON t2.EquipmentTypeID = t1.EquipmentTypeID
WHERE IPAddress LIKE '%192.%'
GROUP BY
t3.IPAddress,
t3.EquipmentID,
t2.EquipmentTypeID,
t2.DisplayName

我认为你应该

加入
T3到T1，然后加入T1到T2

像这样：

select * from T3
Join T1 on T3.EquipmentID = T1.EquipmentID
Join T2 on T2.EquipmentTypeID = T1.EquipmentTypeID 
WHERE T3.IPAddress LIKE '%192'

您两次加入表2，但使用的id错误：

JOIN Table2 t3 ON t3.EquipmentTypeID = t1.EquipmentTypeID
JOIN Table2 t4 ON t3.EquipmentTypeID = t1.EquipmentTypeID

我认为你应该
加入
T3到T1，然后加入T1到T2

像这样：

select * from T3
Join T1 on T3.EquipmentID = T1.EquipmentID
Join T2 on T2.EquipmentTypeID = T1.EquipmentTypeID 
WHERE T3.IPAddress LIKE '%192'

你为什么两次加入表2？这有什么道理吗？另外，如果使用“%192”，您将无法得到您显示给我们的结果。为什么要两次加入table2？这有什么道理吗？另外，对于像“%192”这样的问题，您不会得到您显示给我们的结果。谢谢您的帮助：）它起作用了，当我有多个正确答案时，我真的不知道该选择什么：）非常感谢：）您在代码中使用的答案是一个明显的选择：）很高兴我能提供帮助：）谢谢您的帮助：）它起作用了，当我有多个正确答案时，我真的不知道该选择什么：）非常感谢：）你在代码中使用的答案是一个明显的选择：）很高兴我能帮忙：）