如何創建有條件的LEFT JOIN？

Question

我想LEFT JOIN 3個表，像這樣：

DECLARE @CustomerID AS INT; 
DECLARE @ProductID AS INT; 

SELECT * 
FROM table1 t1 
    LEFT JOIN table2 t2 ON t1.id = t2.id 
    LEFT JOIN table3 t3 ON t2.loc = t3.loc 
WHERE t1.id = @ProductID 
    AND (t2.loc = t3.loc OR t2.loc IS NULL) 
    AND (t3.cid = @CustomerID OR t3.cid IS NULL) 

有4個，我想解決的基本情況：

1. @CustomerID <> 0，@ProductID存在於T1僅
2. @CustomerID <> 0和@ProductID存在於t1和t2
3. @CustomerID = 0和@ProductID存在於T1僅
4. @客戶ID = 0和@ProductID存在於T1和T2

上面的代碼工作的情況下1-3，但萬一4.返回什麼，我想這是因爲最後的LEFT JOIN中斷（即使這兩個數據存在對於@ProductID，t1和t2）。

有沒有辦法讓第二個LEFT JOIN條件不使用IF ... ELSE邏輯？

Answer 1

把條件on子句中，而不是where條款

SELECT * 
FROM table1 t1 
    LEFT JOIN table2 t2 ON t1.id = t2.id 
    LEFT JOIN table3 t3 ON t2.loc = t3.loc 
         AND (t3.cid = @CustomerID OR t3.cid IS NULL) 
         AND (t2.loc = t3.loc OR t2.loc IS NULL) 
WHERE t1.id = @ProductID 

Answer 2

如果我知道你想什麼，這可能工作：

SELECT * FROM table1 a 
    LEFT JOIN table2 b 
     ON b.id = a.id 
    LEFT JOIN table3 c 
     ON c.loc = b.loc 
     and isNull(c.cid, @CustomerID) = CustomerID 
WHERE t1.id = @ProductID