在MySQL中有多个连接的group_concat

Question

数据库架构

create table `questions` (
  `id` int not null auto_increment,
  `title` varchar(45) not null,
  primary key (`id`));

create table `tags` (
  `id` int not null auto_increment,
  `question_id` int not null,
  `name` varchar(45) not null,
  primary key (`id`));

create table `comments` (
  `id` int not null auto_increment,
  `question_id` int not null,
  `body` varchar(45) not null,
  primary key (`id`));

insert into questions (title) values
("title1"), ("title2"), ("title3");

insert into tags (question_id, name) values
(1, "javascript"), (1, "php"), (1, "c#"), (2, "mysql"), (2, "php"), (3, "c#");

insert into comments (question_id, body) values
(1, "comment1"), (1, "comment1"), (1, "comment2"), (3, "comment3");

这就是它的视觉效果:

questions 表

| id |  title |
|----|--------|
|  1 | title1 |
|  2 | title2 |
|  3 | title3 |

tags 表

| id | question_id |       name |
|----|-------------|------------|
|  1 |           1 | javascript |
|  2 |           1 |        php |
|  3 |           1 |         c# |
|  4 |           2 |      mysql |
|  5 |           2 |        php |
|  6 |           3 |         c# |

comments 表

| id | question_id |     body |
|----|-------------|----------|
|  1 |           1 | comment1 |
|  2 |           1 | comment1 |
|  3 |           1 | comment2 |
|  4 |           3 | comment3 |

每个问题必须至少有一个标签.它还可以有0个或更多注释.在一个问题上,同一个机构可以有两条评论.

期望的输出

我想选择所有问题,即他们的ID,标题,标签和评论.

输出应该如下所示:

| id |  title |       tags        |          comments          |
|----|--------|-------------------|----------------------------|
|  1 | title1 | c#,php,javascript | comment1,comment1,comment2 |
|  2 | title2 | php,mysql         | (null)                     |
|  3 | title3 | c#                | comment3                   |

试图解决问题

我尝试了以下查询:

select questions.id, questions.title,
  group_concat(tags.name), group_concat(comments.body)
from questions
join tags on questions.id = tags.question_id
left join comments on questions.id = comments.question_id
group by questions.id

不幸的是,它没有按预期工作.它产生以下输出:

| id |  title | group_concat(distinct tags.name) |                                                      group_concat(comments.body) |
|----|--------|----------------------------------|----------------------------------------------------------------------------------|
|  1 | title1 |                c#,php,javascript | comment1,comment1,comment1,comment2,comment2,comment2,comment1,comment1,comment1 |
|  2 | title2 |                        php,mysql |                                                                           (null) |
|  3 | title3 |                               c# |                                                                         comment3 |

如您所见,对于第一个问题,我每次都会收到三次评论,因为此问题上有三个标记.

此外,评论的顺序错误.它们应该与插入的顺序相同,即comment1,comment1,comment2不是comment1,comment2,comment1.

我不能distinct用于评论,因为在一个问题上可以有同一个机构的多个评论.

我知道这可能可以用嵌套select的方法来解决,但据我所知,它会对查询的性能产生巨大的负面影响.

SQL小提琴

SQL Fiddle with database schema和我的查询.

Answer 1

您需要先汇总和应用GROUP_CONCAT,并随后加入:

select questions.id, questions.title,
       tags.name, comments.body
from questions
join (
   select question_id, group_concat(tags.name) as name
   from tags
   group by question_id
) tags on questions.id = tags.question_id
left join (
   select question_id, group_concat(comments.body) as body
   from comments
   group by question_id
) comments on questions.id = comments.question_id