Question

我有'发布'和'评论'表。我想选择最后3个帖子和对该帖子的所有评论。我可以使用2个单独的语句：

SELECT p.* FROM post p ORDER BY p.date DESC LIMIT 3; // called 1
SELECT c.* FROM comment c WHERE c.post_id = :id; // called 3x time for each post.

可以将这些查询归为一个吗？

Answer 1

您可以将子查询用于帖子集：

SELECT p.*
FROM (SELECT p.*
      FROM post p
      ORDER BY p.date DESC
      LIMIT 3
     ) p JOIN
     comment c
     ON c.post_id = p.id
ORDER BY p.id, c.id;

Answer 2

SELECT   POST .*, COMMENT.* 
from POST INNER JOIN COMMENT ON POST.id = COMMENT.post_id
where  POST.id = COMMENT.post_id ORDER BY POST.id  LIMIT 3

Answer 3

你也可以尝试这个

SELECT p . * , c . * 
FROM post p
LEFT JOIN 
COMMENT c ON c.post_id = p.id
where p.id IN (SELECT id from post order by date desc limit 3) ORDER BY p.date

LIMFT行与LEFT连接

3 个答案: