使用COUNT和HAVING BY进行SELECT查询

Question

How can I modify the below query to limit the number of rows fetched PER userid from the post_list table without affecting the performance of the script (the post_list table is over 10,000 rows here) ?

$sql = query("SELECT userid, post, post_id, id, username 
FROM post_list, users
WHERE post_list.userid = users.id AND post_id%4 = 0 
ORDER BY post_id"); 

I need to have 5 posts MAX per post_list.userid.

So if i have 10 users in my post_list table , the query will return 50 rows.

EDIT:

USERS table

  id        username
   1          bill
   2          mark
   3          kate

POST_LIST table

 post_id    userid    post 
   1           1       foo
   2           3       bar
   3           3       baz
   4           1       qux
   5           1       foo_1
   6           1       bar_1
   7           1       baz_1
   8           1       qux_1

Here, the query should only return 5 posts from Bill.

Also, i need the overall number of rows to be limited to 50.

Answer 1

Use LIMIT

ORDER BY post_id LIMIT 5;

---

As you commented, you can take a look at this article here which will explain you in detail, How to select the first/least/max row per group in SQL .

Answer 2

SELECT pl.*, u.*
FROM users AS u
JOIN (
    SELECT 
        userid,
        SUBSTRING_INDEX(GROUP_CONCAT(post_id ORDER BY post_id DESC), ',', 5) AS last_5_posts
    FROM post_list
    GROUP BY userid
) AS pl5 ON(u.id = pl5.userid)
JOIN post_list AS pl ON (FIND_IN_SET(pl.post_id, pl5.last_5_posts))
ORDER BY pl.post_id DESC
LIMIT 50

Example: http://sqlfiddle.com/#!2/36f12/30

Answer 3

You can do this by group by and count. Please try this query.

$sql = query("SELECT userid, post, post_id,username, wish_likes FROM post_list, 
users WHERE post_list.userid = users.id AND post_id%4 = 0 group by userid,post,   
 post_id,username, wish_likes having 
count(post_id) <= 5 ORDER BY post_id");