SQL Server：加入新语法（ANSI 与非 ANSI SQL JOIN 语法）

Question

我尝试将旧的 MS sql 连接语法转换为新的连接语法，但结果中的行数不匹配。

原始SQL：

select  
    b.Amount
from 
    TableA a, TableB b,TableC c, TableD d 
where 
    a.inv_no *= b.inv_no and 
    a.inv_item *= b.inv_item and 
    c.currency *= b.cash_ccy and
    d.tx_code *= b.cash_receipt

转换后的 SQL：

SELECT
    b.AMOUNT
FROM
    (TableA AS a 
LEFT OUTER JOIN
    TableB AS b ON a.INV_NO = b.INV_NO 
                AND a.inv_item = b.inv_item 
LEFT OUTER JOIN 
    TableC AS c ON c.currency = b.cash_ccy)
LEFT OUTER JOIN
    TableD as d ON d.tx_code = b.cash_receipt

调查结果

原始 SQL 和修改后的 SQL 最多连接 3 个表的结果相同，但将第四个表 (TableD) 连接到修改后的 SQL 时，返回的行数不同。

Answer 1

在使用 SQL Server（已弃用）专有的 ANSI 89 连接语法 *= 或 =* 时，谓词中的字段顺序很重要

所以当

SELECT  *
FROM    TableA AS A
        LEFT JOIN TableB AS B
            ON A.ColA = B.ColB;

完全一样

SELECT  *
FROM    TableA AS A
        LEFT JOIN TableB AS B
            ON B.ColB = A.ColA;     -- NOTE ORDER HERE

等价

SELECT  *
FROM    TableA AS A, TableB AS b
WHERE   A.ColA *= B.ColB;

不一样

SELECT  *
FROM    TableA AS A, TableB AS b
WHERE   B.ColA *= A.ColB;

最后一个查询的 ANSI 92 等效项是

SELECT  *
FROM    TableA AS A
        RIGHT JOIN TableB AS B
            ON A.ColA = B.ColB;

或者如果你和我一样不喜欢RIGHT JOIN，你可能会写：

SELECT  *
FROM    TableB AS B
        LEFT OUTER JOIN TableA AS A
            ON B.ColB = A.ColA;

所以实际上 ANSI 92 连接语法中的等效查询将涉及从 TableA、TableC 和 TableD 开始（因为这些是原始 WHERE 子句中的前导字段）。那么由于三者之间没有直接联系，你最终得到了一个交叉连接

SELECT  b.Amount
FROM    TableA AS a     
        CROSS JOIN TableD AS d
        CROSS JOIN TableC AS c
        LEFT JOIN TableB AS B
            ON c.currency = b.cash_ccy
            AND d.tx_code = b.cash_receipt
            AND a.INV_NO = b.INV_NO 
            AND a.inv_item = b.inv_item;

这是等效的重写，并解释了行数的差异

工作示例

^{需要在 SQL Server 2008 或更早版本上运行，兼容级别为 80 或更低}

-- SAMPLE DATA -- 
CREATE TABLE #TableA (Inv_No INT, Inv_item INT);
CREATE TABLE #TableB (Inv_No INT, Inv_item INT, cash_ccy INT, cash_receipt INT, Amount INT);
CREATE TABLE #TableC (currency INT);
CREATE TABLE #TableD (tx_code INT);

INSERT #TableA (inv_no, inv_item) VALUES (1, 1), (2, 2);
INSERT #TableB (inv_no, inv_item, cash_ccy, cash_receipt, Amount) VALUES (1, 1, 1, 1, 1), (2, 2, 2, 2, 2);
INSERT #TableC (currency) VALUES (1), (2), (3), (4);
INSERT #TableD (tx_code) VALUES (1), (2), (3), (4);

-- ORIGINAL QUERY(32 ROWS)
SELECT  
    b.Amount
FROM 
    #TableA a, #TableB b,#TableC c, #TableD d 
WHERE 
    a.inv_no *= b.inv_no and 
    a.inv_item *= b.inv_item and 
    c.currency *= b.cash_ccy and
    d.tx_code *= b.cash_receipt

-- INCORRECT ANSI 92 REWRITE (2 ROWS)
SELECT  b.AMOUNT
FROM    #TableA AS a 
        LEFT OUTER JOIN #TableB AS b 
            ON a.INV_NO = b.INV_NO 
            and a.inv_item = b.inv_item 
        LEFT OUTER JOIN #TableC AS c 
            ON c.currency = b.cash_ccy
        LEFT OUTER JOIN #TableD as d 
            ON d.tx_code = b.cash_receipt;


-- CORRECT ANSI 92 REWRITE (32 ROWS)
SELECT  b.Amount
FROM    #TableA AS a        
        CROSS JOIN #TableD AS d
        CROSS JOIN #TableC AS c
        LEFT JOIN #TableB AS B
            ON c.currency = b.cash_ccy
            AND d.tx_code = b.cash_receipt
            AND a.INV_NO = b.INV_NO 
            AND a.inv_item = b.inv_item;