mysql - 如何计算所有行并为结果集中不存在的行返回 0

Question

我有这个 MySQL 模式 Mysql Fiddle .

GRPS                        GRP_USRS
grpID | name                usrID | grpID  
------------                --------------
1     | A                   1     | 1
2     | B                   2     | 1
3     | C                   3     | 1
4     | D                   4     | 1
5     | E                   16    | 2
                            11    | 3
                            12    | 3

我的目标是获取所有用户总数的组，包括 0。 像这样:

grpID | count(*)
--------------
1     | 4
2     | 1
3     | 2
4     | 0
5     | 0

我试过:

SELECT grps.grpID, IFNULL(COUNT(grp_usrs.usrID), 0) AS users
FROM grps 
     INNER JOIN grp_usrs ON grps.grpID = grp_usrs.grpID
GROUP BY grps.grpID ASC;

但结果是:

grpID | count(*)
--------------
1     | 4
2     | 1
3     | 2

如何包含计数为 0 的对象

Answer 1

你几乎做对了。你应该使用左连接:

SELECT grps.grpID, COUNT(grp_usrs.usrID) AS users
FROM grps 
LEFT JOIN grp_usrs ON grps.grpID = grp_usrs.grpID
GROUP BY grps.grpID ASC;