MySQL在同一个表中计数,包括零计数值

Question

我有这个表结构与数据:

INSERT INTO `test` (`id`, `email`, `id_user_ref`, `name`) VALUES
(1, 'email@gmail.com', NULL, 'Mike'),
(2, 'email2@gmail.com', '1', 'Jhonny'),
(3, 'email3@gmail.com', '1', 'Michael'),
(4, 'email4@gmail.com', '2', 'Jorhe'),
(5, 'email5@gmail.com', '3', 'Mia');

我需要使用此查询计算所有用户的id_user_ref:

SELECT id, COUNT(name) AS refNr FROM test GROUP BY id_user_ref
HAVING id_user_ref IS NOT NULL;

这有效,但问题是我需要显示所有结果,即使计数结果为0.

我尝试了几个与同一个表的左连接,但没有任何成功.

输出应该是:

id  refNr
1    2
2    1
3    1
4    0
5    0

Answer 1

试试这个:

SELECT 
  t1.id, 
  IFNULL(COUNT(t2.name), 0) AS refNr 
FROM test AS t1
LEFT JOIN test AS t2 ON t1.id = t2.id_user_ref
GROUP BY t1.id;

SQL Fiddle DEmo

这会给你:

| ID | REFNR |
--------------
|  1 |     2 |
|  2 |     1 |
|  3 |     1 |
|  4 |     0 |
|  5 |     0 |