如何将“select count”添加为列

Question

我的表格中有invite_code和invited_by列。

我想选择所有用户以及此用户邀请的用户数。这可以在一个查询中完成吗？

e.g。我有这些数据：

id | invite_code | invited_by   
-----------------------------     
1  |          11 | 
2  |          22 |        11
3  |          33 |        11
4  |          44 |        22

我想在结果中添加select count(*) from users where u.invite_code = invited_by：

id | invite_code | invited_by | invite_num
------------------------------------------
1  |          11 |            |         2
2  |          22 |         11 |         1
3  |          33 |         11 |         0
4  |          44 |         22 |         0

我正在使用MySQL

Answer 1

试试这个：

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Answer 2

您也可以使用连接而不是列列表中的子查询：

select u1.id, u1.invite_code, u1.invited_by, u2.invite_num
from users u1
left join (select invited_by, count(*) as invite_num from users group by invited_by) u2
     on u1.invite_code = u2.invited_by;

Answer 3

最有效的方法是使用自我加入：

select u.id, u.invite_code, u.invited_by, count(u2.id) as invite_num
from user u
left outer join user u2 on u2.invited_by = u.invite_code
group by u.id, u.invite_code, u.invited_by