SELECT 3 records per user group by on two columns

问题

I've been stuck in a complex MySQL query. Here is my table:

+--------------------------------------+
| id | user_id | category_id | post_id |
+--------------------------------------+
| 1  | 23      | 5           | 213     |
| 2  | 23      | 5           | 214     |
| 3  | 23      | 5           | 215     |
| 4  | 23      | 5           | 216     |
| 5  | 23      | 6           | 217     |
| 6  | 23      | 6           | 218     |
| 7  | 23      | 6           | 219     |
| 8  | 23      | 6           | 220     |
| 9  | 55      | 13          | 221     |
| 10 | 55      | 13          | 222     |
| 11 | 55      | 16          | 223     |
| 12 | 55      | 16          | 234     |
| 13 | 55      | 22          | 235     |
| 14 | 55      | 22          | 256     |
| 15 | 55      | 22          | 261     |
| 16 | 62      | 13          | 272     |
| 17 | 62      | 13          | 273     |
| 18 | 62      | 24          | 277     |
| 19 | 62      | 24          | 278     |
| 20 | 62      | 24          | 288     |
| 21 | 62      | 31          | 289     |
| 22 | 62      | 31          | 290     |
+--------------------------------------+

Now what I wish is for each user_id I want 2 rows of data but each row should have a different category_id, like the below resultset:

+--------------------------------------+
| id | user_id | category_id | post_id |
+--------------------------------------+
| 1  | 23      | 5           | 213     |
| 5  | 23      | 6           | 217     |
| 9  | 55      | 13          | 221     |
| 11 | 55      | 16          | 223     |
| 16 | 62      | 13          | 272     |
| 18 | 62      | 24          | 277     |
+--------------------------------------+

The query I've used so far using GROUP BY clause only manages to return a single row for each group, but I want 2 or possibly 3. Here is my query:

SELECT * FROM (
    SELECT id, user_id, category_id, post_id 
    FROM my_table
    GROUP BY user_id, category_id) 
AS sub GROUP BY sub.user_id;

Please suggest how to go from here...

回答1:

You can use user-defined variables to give rank for the same user group and in outer query you can simple use your condition for showing 2 categories per user or 3 or more as you need to

SELECT id,
  user_id,
  category_id,
  post_id,
  rank FROM 
(SELECT tt.* ,
@rank:= CASE WHEN @group = user_id THEN @rank + 1 ELSE 1 END rank,
@group:= tt.user_id
FROM
(SELECT 
  id,
  user_id,
  category_id,
  post_id 
FROM
  table_name 
GROUP BY user_id,
  category_id
  ORDER BY user_id,
  category_id 
  ) tt
 JOIN (SELECT @rank:=0,@group:=0) t1
 ) new_t
 WHERE rank <=2 /* will give 2 records per user change to 3 if you need to show 3 records per user */

Demo 2 Rows Per User

Demo 3 Rows Per User

来源：https://stackoverflow.com/questions/24139628/select-3-records-per-user-group-by-on-two-columns

标签

mysql

sql

greatest-n-per-group