LIMIT行左加入

Question

我有'發佈'和'評論'表。我想選擇最後3個帖子和對該帖子的所有評論。我可以使用兩個單獨的語句：

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. 

可以將這些查詢合併爲一個？

Answer 1

您可以使用子查詢了一套崗位：

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

你也可以試試這個

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