MySQL Join三个表返回多个结果

admin

MySQL Join三个表返回多个结果

sql

我有三个表：我们称其为CUSTOMER，LOG和REVIEW

客户表为：

id name
== ====
1  John
2  Jane
3  Mike

LOG表是

id customer_id  created_at
== ===========  ==========
1  1            2015-06-10
2  1            2015-06-10
3  2            2015-06-11
4  1            2015-06-13
5  2            2015-06-15
6  1            2015-06-15

REVIEW表是

id customer_id  created_at
== ===========  ==========
1  1            2015-06-10
2  2            2015-06-10
3  2            2015-06-11
4  1            2015-06-13
5  1            2015-06-15
6  1            2015-06-15
7  1            2015-06-18

我想要什么

CUSTOMER_ID NAME LOG_QTY REVIEW_QTY
=========== ==== ======= ==========
1           John 4       5
2           Jane 2       2
3           Mike 0       0

我得到了：

CUSTOMER_ID NAME LOG_QTY REVIEW_QTY
=========== ==== ======= ==========
1           John 20      20
2           Jane 4       4
3           Mike 0       0

我的查询：

                       select CUSTOMER.ID, CUSTOMER.NAME,
 count(REVIEW.CUSTOMER_ID) as REVIEW_QTY,
    count(LOG.CUSTOMER_ID) as LOG_QTY
                         from CUSTOMER
                    left join REVIEW
                           on REVIEW.CUSTOMER_ID = CUSTOMER.ID
                    left join LOG
                           on LOG.CUSTOMER_ID = CUSTOMER.ID
                     group by CUSTOMER.ID
                     order by CUSTOMER.ID

阅读 201

2021-06-07

共1个答案

admin

如果您在不使用COUNT()和GROUP BY的情况下运行查询，则会看到发生了什么：

select CUSTOMER.ID, CUSTOMER.NAME,
 REVIEW.CUSTOMER_ID as REVIEW_QTY,
 LOG.CUSTOMER_ID as LOG_QTY
from CUSTOMER
 left join REVIEW on REVIEW.CUSTOMER_ID = CUSTOMER.ID
 left join LOG on LOG.CUSTOMER_ID = CUSTOMER.ID
order by CUSTOMER.ID

这将为每个可能的行组合返回一行，并CUSTOMER_ID从三个表中返回相同的行（就是INNER JOIN这样）。然后COUNT就算是他们！

这应该给您您需要的：

select CUSTOMER.ID, CUSTOMER.NAME,
 (select count(*) from REVIEW where CUSTOMER_ID = CUSTOMER.ID) as REVIEW_QTY,
 (select count(*) from LOG where CUSTOMER_ID = CUSTOMER.ID)  as LOG_QTY
from CUSTOMER
order by CUSTOMER.ID

2021-06-07