Question

以下是一个示例查询：

SELECT u.user_id, 
    (SELECT GROUP_CONCAT(photo_id) 
        FROM photos p WHERE p.user_id = u.user_id) 
    AS photo_ids FROM users u;

现在，我们举个例子来说，我们有这些表：

用户：

+---------+-----------------------+
| user_id |         email         |
+---------+-----------------------+
|       1 | webmaster@example.com |
|       2 | admin@example.com     |
+---------+-----------------------+

照片：

+---------+----------+
| user_id | photo_id |
+---------+----------+
|       1 |        1 |
|       1 |        2 |
+---------+----------+

上述查询的结果将形成以下结果：

+---------+-----------+
| user_id | photo_ids |
+---------+-----------+
|       1 | 1,2       |
|       2 | NULL      |
+---------+-----------+

如何使它成为如果photo_ids列为NULL而不返回？

我试过了：

SELECT u.user_id, 
    (SELECT GROUP_CONCAT(photo_id) 
        FROM photos p WHERE p.user_id = u.user_id) 
    AS photo_ids FROM users u
WHERE photo_ids IS NOT NULL;

但这会返回语法错误：

#1054 - Unknown column 'photo_ids' in 'where clause'

我已经尝试了HAVING，因为我已经阅读了它与group_concat字段一起使用。

SELECT u.user_id, 
    (SELECT GROUP_CONCAT(photo_id) 
        FROM photos p WHERE p.user_id = u.user_id) 
    AS photo_ids FROM users u
HAVING photo_ids IS NOT NULL;

结果： 您的SQL语法有错误;查看与您的MySQL服务器版本对应的手册，以获得正确使用的语法＆＃39; HAVING photo_ids IS NOT NULL＆＃39;在第1行

Answer 1

您可以在mysql中使用派生表并过滤掉以仅选择NOT NULL行。

select * from
(
SELECT u.user_id, 
    (SELECT GROUP_CONCAT(photo_id) 
        FROM photos p WHERE p.user_id = u.user_id)  
    AS photo_ids FROM users u
) as DT
where DT.photo_ids not null;

Live Demo

Answer 2

试试这个MySQL查询： -

SELECT u.user_id, 
    (SELECT GROUP_CONCAT(photo_id) 
        FROM photos p WHERE p.user_id = u.user_id) 
    AS photo_ids FROM users u
WHERE photo_ids IS NOT NULL;

Answer 3

试试right join： -

select Users.user_id,GROUP_CONCAT(photo_id) as photo_ids 
from Users right join Photos 
on Users.user_id=Photos.user_id group by Users.user_id

Run query

如何在GROUP_CONCAT上调用NOT NULL

3 个答案: