SQL-Join with NULL-columns

Question

我有以下表格:

Table a
+-------+------------------+------+-----+
| Field | Type             | Null | Key |
+-------+------------------+------+-----+
| bid   | int(10) unsigned | YES  |     |
| cid   | int(10) unsigned | YES  |     |
+-------+------------------+------+-----+

Table b
+-------+------------------+------+
| Field | Type             | Null |
+-------+------------------+------+
| bid   | int(10) unsigned | NO   |
| cid   | int(10) unsigned | NO   |
| data  | int(10) unsigned | NO   |
+-------+------------------+------+

当我想从b中选择a中相应的bid/cid-pair的所有行时,我只需使用自然连接SELECT b.* FROM b NATURAL JOIN a;,一切都很好.

当a.bid或a.cid为NULL时,我想得到另一列匹配的每一行,例如,如果a.bid为NULL,我想要每一行a.cid=b.cid,如果两者都是NULL,我想要b中的每一列.

我天真的解决方案是这样的:

SELECT DISTINCT b.* FROM b JOIN a ON ( ISNULL(a.bid) OR a.bid=b.bid ) AND (ISNULL(a.cid) OR a.cid=b.cid )

有没有更好的方法来做到这一点？

Answer 1

ISNULL函数实际上不符合ANSI标准.是的,您需要检查两列中的空值.编写查询的另一种方法是:

Select Distinct b.*
From b
    Join a
        On ( a.bid = b.bid Or ( a.bid Is Null And b.bid Is Null ) )
            And ( a.cid = b.cid Or ( a.cid Is Null And b.cid Is Null ) )

另一种避免使用Distinct的方法:

Select b.*
From b
Where Exists    (
                Select 1
                From a
                Where  ( a.bid = b.bid Or ( a.bid Is Null And b.bid Is Null ) )
                    And ( a.cid = b.cid Or ( a.cid Is Null And b.cid Is Null ) )
                )

Answer 2

不，差不多就是这样了。

（我通常会重新表述ISNULL(a.bind)为a.bind IS NULLANSI SQL 合规性 FWIW。）