MySQL: How to get five posts from each user? - php

Question

I want to select users from one table, and check for id in the other by setting LIMIT 5 for each.
Here's what I have, but I can't reuse, the a.id in the 2nd union
SELECT a.id AS USER,
p.id AS post_id
FROM (
(SELECT a.id
FROM `user` a
WHERE a.date=...)
UNION
(SELECT p.id
FROM `post` p
WHERE p.user_id=a.id LIMIT 5)`` `) AS post
Thank you, aloha

Answer 1

The query might look like below:
Main Query:
SELECT
t.userID,
t.postID
FROM
(
SELECT
user.id AS userID,
post.id AS postID,
IF (#prev = post.user_id ,#cn := #cn + 1 ,#cn := 0) SL,
#prev := post.user_id
FROM (SELECT #cn := 0, #prev := 0) var,post
INNER JOIN user ON user.id = post.user_id
ORDER BY post.user_id) t
WHERE t.SL < 5;
The query will pick 5 posts for each user.
TEST:
Unable to add an SQL FIDDLE.
So here's some test data with schema to check the query.
DROP TABLE IF EXISTS `post`;
CREATE TABLE `post` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`user_id` int(11) NOT NULL,
PRIMARY KEY (`id`)
);
INSERT INTO `post` VALUES ('1', '71');
INSERT INTO `post` VALUES ('2', '66');
INSERT INTO `post` VALUES ('3', '66');
INSERT INTO `post` VALUES ('4', '71');
INSERT INTO `post` VALUES ('5', '66');
INSERT INTO `post` VALUES ('6', '71');
INSERT INTO `post` VALUES ('7', '71');
INSERT INTO `post` VALUES ('8', '71');
INSERT INTO `post` VALUES ('9', '66');
INSERT INTO `post` VALUES ('10', '66');
INSERT INTO `post` VALUES ('11', '66');
INSERT INTO `post` VALUES ('12', '66');
INSERT INTO `post` VALUES ('13', '71');
INSERT INTO `post` VALUES ('14', '91');
DROP TABLE IF EXISTS `user`;
CREATE TABLE `user` (
`id` int(11) NOT NULL AUTO_INCREMENT,
PRIMARY KEY (`id`)
);
INSERT INTO `user` VALUES ('66');
INSERT INTO `user` VALUES ('71');
INSERT INTO `user` VALUES ('91');
SELECT
*
FROM post;
Result:
id user_id
1 71
2 66
3 66
4 71
5 66
6 71
7 71
8 71
9 66
10 66
11 66
12 66
13 71
14 91
SELECT * FROM user;
Result:
id
66
71
91
Total posts per user query:
SELECT
user_id,
COUNT(*) totalPost
FROM post
GROUP BY user_id;
Result:
user_id totalPost
66 7
71 6
91 1
Final Result:
userID postID
66 2
66 3
66 5
66 12
66 11
71 1
71 13
71 8
71 7
71 6
91 14