Question

假设我有以下三个表，表示帖子被赋予标签的关系（多对多关系）：

create table posts (id integer, content text, primary key (id));
create table tags (tag varchar(30), description text, primary key (tag));
create table post_tags (post_id integer, tag varchar(10),
    primary key (post_id, tag),
    foreign key (post_id) references posts (id),
    foreign key (tag) references tags (tag));

现在假设我想查找所有标签{clever，interesting}，而不是其他帖子。

这是我失败的尝试。它会找到包含{clever，interesting}标签的帖子，但它也会找到包含{聪明，有趣，烦人}或{聪明，有趣，不道德}标签的帖子。

select t1.post_id from post_tags as t1
    inner join post_tags as t2 on t2.post_id=t1.post_id
    where t1.tag='clever' and t2.tag='interesting';

我愿意接受任何改变结构的建议，以使这项操作更容易。但是，我想避免在posts表中添加以逗号分隔的标记列表。

Answer 1

这是一个确切的关系划分问题。

在SQL Server中，well performing方法（假设post_id,tag上有唯一约束）是

SELECT post_id
FROM   post_tags
GROUP  BY post_id
HAVING MIN(CASE
             WHEN Keyword IN ( 'clever', 'interesting' ) THEN 1
             ELSE 0
           END) = 1
       AND SUM(CASE
                 WHEN Keyword IN ( 'clever', 'interesting' ) THEN 1
                 ELSE 0
               END) = 2

所以我不排除在GROUP_CONCAT中使用HAVING的想法。

HAVING GROUP_CONCAT(DISTINCT Keyword ORDER BY Keyword) = 'clever,interesting'

Answer 2

应该可以使用NOT EXISTS，例如，

select t1.post_id
from post_tags as t1
    inner join post_tags as t2 on t2.post_id = t1.post_id
where 
    t1.tag = 'clever' 
and t2.tag = 'interesting' 
and not exists (
    select *
    from post_tags t3
    where 
        t3.tag not in ('clever', 'interesting')
    and t3.post_id = t1.post_id
)

查询以查找具有一组精确标签的帖子（多对多关系）

2 个答案: