Question

我有以下表结构

表1：

+--------+
| foo_id |
+--------+
|      1 |
|      2 |
|      3 |
+--------+

表2：

+--------+--------+
| foo_id | bar_id |
+--------+--------+
|      1 |      1 |
|      1 |      2 |
|      1 |      3 |
|      3 |      2 |
+--------+--------+

现在，我想要一个像这样的输出：

+--------+--------+
| foo_id | bar_id |
+--------+--------+
|      1 | 1      |
|      2 | null   |
|      3 | null   |
+--------+--------+

我想到的是......好像

select table2.foo_id, table2.bar_id 
from table1
left join table2    on table1.foo_id = table2.foo_id
                       and table2.bar_id = 1

但它不太有用，我为foo_id = 1获得3行。

有什么想法吗？

非常感谢！

Answer 1

您只需要更改：

select table2.foo_id, table2.bar_id 
from table1
left join table2    on table1.foo_id = table2.foo_id
                       and table2.bar_id = 1

到

select table1.foo_id, table2.bar_id 
from table1
left join table2    on table1.foo_id = table2.foo_id
                       and table2.bar_id = 1

您错误地选择了错误的表格从

输出foo_id

Answer 2

select table2.foo_id, table2.bar_id 
from table1
left join table2    on table1.foo_id = table2.foo_id
where table2.bar_id = 1 or table2.bar_id is null

如何连接表：列等于值或为空？

2 个答案: