将一列中的两列连接到另一列中的一列，为两列中的每一列返回不同或相同的结果

Question

我有4张桌子可以轻松加入。最后一个连接需要两个列连接到类别表;其中，我想要在同一行中返回不同的值（categoryName）。

所需输出

customerID，adID，categoryselectedID，providedcategoryID，categoryID，categoryName（这一个是正确的），wantedcategoryID，categoryID，categoryName（这个名称与提供的类别ID相同）

表和列

1. 客户：customerID（PK）
2. ads：adID（PK），customerID（FK）
3. categoriesselected：categoryselectedID（PK），adID（FK），providedcategoryID（FK），wantedcategoryID（FK）
4. 类别：categoryID（PK），categoryName

（mysql返回7条记录，除categoryName之外的所有信息对于提供和缺少都是相同的）

SELECT customers.*, ads.*, categoriesselected.categoryselectedID, categoriesselected.offeredcategoryID, offeredcategory.categoryID, offeredcategory.categoryName, categoriesselected.wantedcategoryID, wantedcategory.categoryID, wantedcategory.categoryName

FROM customers
    INNER JOIN ads ON ads.customerId = customers.customerID
    INNER JOIN categoriesselected ON categoriesselected.adID = ads.adID 
    LEFT OUTER JOIN categories AS offeredcategory ON offeredcategory.categoryID = categoriesselected.offeredcategoryID
    LEFT OUTER JOIN categories AS wantedcategory ON wantedcategory.categoryID = categoriesselected.offeredcategoryID

（我也试过UNION和UNION都返回了14条记录7，因为它们提供了7个想要类别而不是只有7条记录在同一行上有不同的categoryName。）

SELECT customers.*, ads.*, categoriesselected.categoryselectedID, categoriesselected.offeredcategoryID, categories.categoryID, categories.categoryName,  'Offered Category' AS Category 
FROM customers
    JOIN ads ON ads.customerId = customers.customerID
    JOIN categoriesselected ON categoriesselected.adID = ads.adID 
    JOIN categories ON categories.categoryID = categoriesselected.offeredcategoryID

UNION ALL

SELECT customers.*, ads.*, categoriesselected.categoryselectedID, categoriesselected.wantedcategoryID, categories.categoryID, categories.categoryName,  'Wanted Category' AS Category 
FROM customers
    JOIN ads ON ads.customerId = customers.customerID
    JOIN categoriesselected ON categoriesselected.adID = ads.adID 
    JOIN categories ON categories.categoryID = categoriesselected.wantedcategoryID

Answer 1

如果我没有弄错，问题出在最后一次加入，这会给你一个不正确的输出。

SELECT customers.*, ads.*, categoriesselected.categoryselectedID, 
    categoriesselected.offeredcategoryID, offeredcategory.categoryID, offeredcategory.categoryName, 
    categoriesselected.wantedcategoryID, wantedcategory.categoryID, wantedcategory.categoryName
FROM customers
    INNER JOIN ads ON ads.customerId = customers.customerID
    INNER JOIN categoriesselected ON categoriesselected.adID = ads.adID 
    LEFT OUTER JOIN categories AS offeredcategory ON offeredcategory.categoryID = categoriesselected.offeredcategoryID
    LEFT OUTER JOIN categories AS wantedcategory ON wantedcategory.categoryID = categoriesselected.wantedcategoryID

请注意，我已将offeredcategoryID的联接查询条件从wantedcategoryID更改为wantedcategory