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@@ -0,0 +1,39 @@
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+# 查询相关
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+## 连表查询
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+### 统计相关(关联统计数量)
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+> 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
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+1. 关联表数据查询
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+2. 表结构
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+a表字段数据
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+| id | total |
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+| --- | --- |
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+| 1 | 23 |
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+| 2 | 33 |
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+b表字段数据
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+| aId |
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+| 1 |
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+| 1 |
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+| 1 |
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+> 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
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+```sql
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+# 查询1
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+select ff.*,count(aId = 1 or null) as pay
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+from
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+(select total from a where id = 1) as ff,
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+b;
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+# 查询2
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+select ff.*,count(aId = 2 or null) as pay
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+from
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+(select total from a where id = 2) as ff,
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+b;
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+```
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+查询相关结果
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+1.结果1
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+| total | pay |
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+| --- | --- |
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+| 23 | 3 |
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+
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+2.结果2
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+| total | pay |
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+| --- | --- |
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+| 33 | 0 |
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