group_concat 在 MySQL 中有多个连接

group_concat with multiple joins in MySQL

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");

视觉上是这样的:

questions table

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

tags table

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

comments table

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

每个问题都必须至少有一个标签。它也可以有 0 个或多个评论。一个问题可以有两条相同的评论body

期望的输出

我要select所有问题,即他们的id、标题、标签和评论。

输出应如下所示:

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

尝试解决问题

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,每个评论我都得到了三次,因为这个问题上有三个标签。

另外,评论的顺序错了。它们的顺序应与插入时的顺序相同,即 comment1,comment1,comment2,而不是 comment1,comment2,comment1.

我不能用 distinct 评论,因为一个问题可以有多个相同 body 的评论。

我知道这可能可以通过嵌套 select 来解决,但据我所知这会对查询性能产生巨大的负面影响。

SQL Fiddle

The SQL Fiddle 使用数据库模式和我的查询。

您需要聚合并应用GROUP_CONCAT然后然后加入:

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

您可以在联接之前使用子查询进行聚合。由于您似乎具有独特的标签,因此您似乎可以避免对标签使用子查询,而只需像您目前所做的那样加入这些标签:-

SELECT questions.id, 
        questions.title,
        GROUP_CONCAT(tags.name ORDER BY tags.id), 
        comments.body
FROM questions
LEFT OUTER JOIN tags ON questions.id = tags.question_id
LEFT OUTER JOIN 
(
   SELECT question_id, 
            GROUP_CONCAT(comments.body ORDER BY id) as body
   FROM comments
   GROUP BY question_id
) comments ON questions.id = comments.question_id
GROUP BY questions.id, 
        questions.title,
        comments.body

您可能会逃避相关子查询。如果您有大量问题,这可能会更好,但会使用 WHERE 子句限制您感兴趣的问题。不利的一面是我不确定 MySQL 是否足够聪明,可以为每个问题执行一次相关子查询,而不是为每次出现的问题执行一次。

SELECT questions.id, 
        questions.title,
        GROUP_CONCAT(tags.name ORDER BY tags.id), 
        (
            SELECT GROUP_CONCAT(comments.body ORDER BY id) 
            FROM comments
            WHERE questions.id = comments.question_id
            GROUP BY question_id
        ) AS body
FROM questions
LEFT OUTER JOIN tags ON questions.id = tags.question_id
GROUP BY questions.id, 
        questions.title,
        body