MySQL:mm 表的结果作为逗号分隔列

Question

我对此 SQL counting all rows instead of counting individual rows 有两个问题:

1.) 为什么第 2 行不在结果列表中。

2.) 在生产中(mysql 5.7)我收到错误

SELECT 列表包含非聚合列，这与 sql_mode=only_full_group_by 不兼容...

MySQL 5.6 架构设置:

CREATE TABLE MM(
  `post_id` int, 
  `tag_id` int
);

CREATE TABLE Post
(
  `post_id` int, 
  `name` varchar(200)
);

CREATE TABLE Tag(
  `tag_id` int, 
  `tagname` varchar(200)
);

Insert into Post values (1, "First Post");
Insert into Post values (2, "Second Post");

Insert into Tag values (1, "sql");
Insert into Tag values (2, "mm relation");
Insert into Tag values (3, "group concat");

Insert into mm values (1, 1);
Insert into mm values (1, 2);
Insert into mm values (1, 3);

查询 1:

Select 
  Post.post_id,
  Post.name, 
  GROUP_CONCAT(t.tagname SEPARATOR ',') as tags 
  from Post 
left join 
  MM on MM.post_id = Post.post_id
left join
  Tag as t on t.tag_id = MM.tag_id

SQL Fiddle:

| post_id |       name |                         tags |
|---------|------------|------------------------------|
|       1 | First Post | sql,mm relation,group concat |

Answer 1

如果您需要将每个帖子的标签分组，您应该使用GROUP BY Post.post_id:

Select 
  Post.post_id,
  Post.name, 
  GROUP_CONCAT(t.tagname SEPARATOR ',') as tags 
  from Post 
left join 
  MM on MM.post_id = Post.post_id
left join
  Tag as t on t.tag_id = MM.tag_id
GROUP BY Post.post_id

您还可以使用DISTINCT来确保您只获得唯一的关键字:http://sqlfiddle.com/#!9/e7c87e/12