2

标题听起来有点奇怪,但我想做的是如下。给定表格

A.a  A.b         B.a  B.c  B.d
---  ---         ---  ---  ---
1    X           1    10   Z
2    Y           1    30   W

我想要一个可能类似于以下内容的查询:

SELECT A.a, B.d FROM A
some kind of JOIN B ON A.a = B.a
...WHERE A.a = :a AND B.c = :c

现在,如果我进行以下替换,我想得到这些答案:

:a  :c       A.a  B.d
--  --       ---  ---
1   10  -->  1    Z
1   20  -->  1    NULL
2   any -->  empty set

我正在使用 MySQL,所以 FULL JOIN 是不可能的。

未解决

以为我自己解决了:

SELECT A.a, CASE WHEN B.c = :c THEN B.d ELSE NULL FROM A 
LEFT JOIN B ON A.a = B.a
WHERE A.a = :a AND B.c = :c

但这给出了:

:a  :c       A.a  B.d
--  --       ---  ---
1   20  -->  1    NULL
             1    10
4

1 回答 1

3

试试这个:

select distinct a1.a, b2.d from A a1
join B b1 on a1.a = b1.a
left join B b2 on a1.a = b2.a and b2.c = 10
where a1.a = 1
于 2012-04-12T06:05:02.523 回答