如何在SQL中强制缺少联接？

Question

我需要离开外部联接两个表，但我也总是想拥有“不匹配”行。示例：

left table:

|--------------|
| *c1* | *key* |
|--------------|
| a1   | a     |
|------|-------|
| b1   | b     |
|--------------|

right table:

|--------------|
| *c2* | *key* |
|--------------|
| a2   | a     |
|--------------|

expected result, joined by *key* column:

|-------------|
| *c1* | *c2* | 
|-------------|
| a1   | a2   |
|------|------|
| a1   | null | <- this row is needed
|------|------|
| b1   | null | <- in case there is no match, only one "value, null" row is needed
|-------------|

实现它的最佳方法是什么？最好不使用distinct或union。

Answer 1

我认为您想要inner join和union all：

select l.c1, r.c2
from left l join
     right r
     on l.key = r.key
union all
select l.c1, null
from left l;