# 查询相关
## 连表查询
### 统计相关(关联统计数量)
> b表为售出的表情况,a表记录了总数量,现在需要查询指定商品2的售出情况.应该为 0/33
1. 关联表数据查询
2. 表结构
a表字段数据
| id | total | 
| --- | --- |
| 1 | 23 |
| 2 | 33 |
b表字段数据
| aId |
| 1 |
| 1 |
| 1 | 
> 查询思路,使用`count`函数进行查询,count中会统计所有非null的值,所以判断后应该将false转换为null
```sql
# 查询1
select ff.*,count(aId = 1 or null) as pay
from 
(select total from a where id = 1) as ff,
b;
# 查询2
select ff.*,count(aId = 2 or null) as pay
from 
(select total from a where id = 2) as ff,
b;
```
查询相关结果
1.结果1
| total | pay |
| --- | --- |
| 23 | 3 |

2.结果2
| total | pay |
| --- | --- |
| 33 | 0 |