mysql - 选择 Join table with table self

Question

我有问题...

Table : history
|id   |transaction|created_at         |merchant_id|
|-----|-----------|-------------------|-----------|
|1    |400        |2015-10-12 11:08:37|33         |
|1    |500        |2015-10-15 09:38:22|33         |
|1    |600        |2015-10-21 14:47:12|22         |
|2    |100        |2015-09-26 10:48:27|31         |
|2    |500        |2015-09-30 11:18:07|27         |
|2    |300        |2015-10-02 17:33:57|31         |

我想在我查询时:

SELECT SUM(a.transaction)/COUNT(a.transaction) AS avg_trans
FROM history AS a GROUP BY a.id, a.merchant_id

Result:
|id    |avg_trans|merchant_id|
|------|---------|-----------|
|1     |450      |33         |
|1     |600      |22         |
|2     |200      |31         |
|2     |500      |27         |

然后将 avg_trans 显示到表格历史记录中，如下所示:

|id   |transaction|created_at         |avg_trans|merchant_id|
|-----|-----------|-------------------|---------|-----------|
|1    |400        |2015-10-12 11:08:37|450      |33         |
|1    |500        |2015-10-15 09:38:22|450      |33         |
|1    |600        |2015-10-21 14:47:12|600      |22         |
|2    |100        |2015-09-26 10:48:27|200      |31         |
|2    |200        |2015-09-30 11:18:07|500      |27         |
|2    |300        |2015-10-02 17:33:57|200      |31         |

谁能帮帮我？

Answer 1

正如@Strawberry 指出的那样，您需要使用子查询来计算 id 和商家 id 的平均值，并将其加入到您的主表中:

select t1.*, t2.avg_trans
from table t1
inner join (select id, merchant_id, avg(transaction) avg_trans
            from table
            group by id, merchant_id) t2 on t1.id=t2.id and t1.merchant_id=t2.merchant_id