Postgresql:LEFT JOIN不起作用

时间:2016-05-05 10:50:00

标签: postgresql many-to-many left-join

我有两个表tb1和tb2,其中包含多对多关系:

tb1:
id  | code
-------------
1   | 6%
2   | 12%
tb2:
id  | code
-------------
1   | code_001
2   | code_002
tb2_has_tb1:
id_tb2  | id_tb1   | money
---------------------------
1       | 1        | 250.0
1       | 2        | 300.0
2       | 1        | 100.0

我想得到的结果是:

code_tb2  |code_tb1  | money
------------------------------
code_001  |6%        | 250.0
code_001  |12%       | 300.0
code_002  |6%        | 100.0
code_002  |12%       | 0.0   //----I want to display this row with zero if there is no join

所以我所做的是使用左连接的选择查询,如下所示:

SELECT tb2.code as code_tb2, tb1.code as code_tb1, money
FROM tb1 LEFT JOIN tb2_has_tb1 ON (tb1.id = id_tb1)
LEFT JOIN tb2 ON (tb2.id = id_tb2)

但我一直得到这个结果:

code_tb2  |code_tb1  | money
------------------------------
code_001  |6%        | 250.0
code_001  |12%       | 300.0
code_002  |6%        | 100.0

我错过了什么?

1 个答案:

答案 0 :(得分:1)

由于您希望保留tb1tb2的所有可能的ID值组合,因此一个选项是CROSS JOIN这些表。然后,LEFT JOIN将此结果发送到桥接表tb2_has_tb1以保留每个关系。

SELECT tb2.code AS code_tb2, tb1.code AS code_tb1,
    COALESCE(t.money, 0.0) AS money
FROM tb1 CROSS JOIN tb2
LEFT JOIN tb2_has_tb1 t
    ON tb1.id = t.id_tb1 AND tb2.id = t.id_tb2