使用多表连接获取表行数答案

【问题标题】：Get table row count using multiple table join使用多表连接获取表行数
【发布时间】：2019-06-28 05:28:31
【问题描述】：

我有三张桌子company_m，client_m，companyuser_m

company_m

 CompanyID  CompName    
----------+-----------
     7        Comp3 
     8        Comp2 
     9        Comp1 
     10       Company 
     11       Company4

client_m

clid    AttachToCompanyID   clname
-----+--------------------+------
  1           7             client1
  2           7             client2
  6           8             client3

companyuser_m

uid CompanyID   uname
---+---------+-------
 1    7         user1
 2    10        user2

我的目标

我需要获取对应公司的所有客户和用户的数量

预期输出

CompanyID   CompName    clientcount usercount
---------+-----------+--------------+--------
   7        Comp3          2           1
   8        Comp2          1           0
   9        Comp1          0           0
  10        Company 3      0           1
  11        Company4       0           0

我的代码

select company_m.CompanyID,company_m.CompName,
count(client_m.AttachToCompanyID) as clientcount ,
count(companyuser_m.CompanyID) as usercount from company_m
left join client_m on client_m.AttachToCompanyID=company_m.CompanyID
left join companyuser_m on companyuser_m.CompanyID=company_m.CompanyID
group by company_m.CompanyID

但是上面的代码没有给出正确的输出。

查看我的fiddle

【问题讨论】：

标签： mysql join rowcount

【解决方案1】：

您应该加入单独的子查询，这些子查询在各自的 client_m 和 companyuser_m 表中找到计数：

SELECT
    t1.CompanyID,
    t1.CompName,
    COALESCE(t2.cnt, 0) AS client_count,
    COALESCE(t3.cnt, 0) AS user_count
FROM company_m t1
LEFT JOIN
(
    SELECT AttachToCompanyID, COUNT(*) AS cnt
    FROM client_m
    GROUP BY AttachToCompanyID
) t2
    ON t1.CompanyID = t2.AttachToCompanyID
LEFT JOIN
(
    SELECT CompanyID, COUNT(*) AS cnt
    FROM companyuser_m
    GROUP BY CompanyID
) t3
    ON t1.CompanyID = t3.CompanyID;

【讨论】：