將一個表中的兩列連接到其他表中的一列，則兩列中的每一個都返回不同或相同的結果

Question

我有4張桌子，可以輕松加入。 最后一個聯接要求將兩列聯接到一個類別表中。 我想要在同一行中返回不同的值（categoryName）。

期望的輸出

customerID，adID，categoryselectedID，officecategoryID，categoryID，categoryName（這是正確的），wantcategoryID，categoryID，categoryName（這與顯示的名稱相同）

表和列

1. 客戶：customerID（PK）
2. 廣告：adID（PK），customerID（FK）
3. 所選類別：categoryselectedID（PK），adID（FK），提供的categoryID（FK），想要的categoryID（FK）
4. 類別：categoryID（PK），categoryName

（mysql返回7條記錄，除categoryName之外，所有信息均與提供和需求相同）

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

（我還嘗試了UNION和UNION都返回14條記錄7，以提供7種想要的類別，而不是只有7條記錄在同一行上具有不同的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

如果我沒記錯的話，問題就出在最后一個聯接上，這給了你錯誤的輸出。

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

請注意，我改變了聯接查詢條件從offeredcategoryID到wantedcategoryID為wantedcategory