如何使用多個條件執行左連接？

Question

我有值的表和1標識符和我想LEFT JOIN識別符表中的值表。

的問題是，在標識表，它不是1：1匹配。

我需要的左連接到首先匹配由成本中心的一切，但具有連接特定的科目類別的那些，我想只有那些賬戶類的成本中心內進行映射。

Tables

我將如何構建我的連接語句？

目前，我有它設置爲：

`SELECT * FROM [TABLE 1] t1 
LEFT JOIN [TABLE 2] t2 
ON t1.[Cost Center] = t2.[Cost Center] 
AND **NEED SOMETHING TO FILTER OUT SELECTED ACCOUNT CATEGORIES` 

Answer 1

您可以添加在加入或地方的條件。

SELECT * FROM [TABLE 1] t1 
LEFT JOIN [TABLE 2] t2 
ON t1.[Cost Center] = t2.[Cost Center] 
AND t1.Category = 'bleh' 

或

SELECT * FROM [TABLE 1] t1 
LEFT JOIN [TABLE 2] t2 
ON t1.[Cost Center] = t2.[Cost Center] 
WHERE t1.Category = 'bleh' 

Answer 2

，如果我的理解這個問題正確的，這應該工作：

SELECT t1.[Cost Center], t1.[Account Category],t2.[Flag] 
FROM [TABLE 1] t1 
LEFT JOIN [TABLE 2] t2 
ON t1.[Cost Center] = t2.[Cost Center] 
AND t1.[Account Category] = t2.[Account Category] 

Answer 3

如果我正確理解你的問題，這裏的技巧是，你需要2點LEFT JOIN聲明並在選擇ISNULL。下面，第二個LEFT JOIN只查看沒有AccountCategory的行。這會產生你的Table3。該解決方案假設Table1的AccountCategory不能爲空/空。

設置：

DECLARE @Table1 TABLE 
(
    CostCenter INT, 
    AccountCategory VARCHAR(10) 
) 

DECLARE @Table2 TABLE 
(
    CostCenter INT, 
    AccountCategory VARCHAR(10), 
    Flag VARCHAR(10) 
) 

INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8601, 'ABC1'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8601, 'ABC1'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8601, 'ABC2'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8601, 'ABC3'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8601, 'ABC4'); 

INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8602, 'ABC1'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8602, 'ABC1'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8602, 'ABC2'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8602, 'ABC3'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8602, 'ABC4'); 

INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8603, 'ABC1'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8603, 'ABC1'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8603, 'ABC2'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8603, 'ABC3'); 
INSERT INTO @Table1 (CostCenter, AccountCategory) VALUES (8603, 'ABC4'); 

INSERT into @Table2 (CostCenter, AccountCategory, Flag) VALUES (8601, 'ABC1', 'Yes'); 
INSERT into @Table2 (CostCenter, AccountCategory, Flag) VALUES (8602, null, 'Yes'); 
INSERT into @Table2 (CostCenter, AccountCategory, Flag) VALUES (8603, null, 'Yes'); 

查詢：

SELECT t1.CostCenter, t1.AccountCategory, ISNULL(t2a.Flag, t2b.Flag) AS Flag 
FROM @Table1 t1 
LEFT JOIN @Table2 t2a ON ISNULL(t2a.AccountCategory,'') = t1.AccountCategory AND t1.CostCenter = t2a.CostCenter 
LEFT JOIN @Table2 t2b ON ISNULL(t2b.AccountCategory,'') = '' AND t1.CostCenter = t2b.CostCenter 

結果：

CostCenter AccountCategory Flag 
8601 ABC1 Yes 
8601 ABC1 Yes 
8601 ABC2 NULL 
8601 ABC3 NULL 
8601 ABC4 NULL 
8602 ABC1 Yes 
8602 ABC1 Yes 
8602 ABC2 Yes 
8602 ABC3 Yes 
8602 ABC4 Yes 
8603 ABC1 Yes 
8603 ABC1 Yes 
8603 ABC2 Yes 
8603 ABC3 Yes 
8603 ABC4 Yes