mysql retrive data giving limit and sub limit - mysql

Question

I have a table named item with category ID and item IDs.
Item ID is the primary key and each item is related to category ID.
I want to retrieve 20 items, but 4 items for one category. If there aren't 4 items for
some category, then select all items from this category.
This is my query, it is not working:
SELECT * FROM `item`
JOIN (SELECT `item_id` FROM `item` GROUP BY `category_id` LIMIT 4) LIMIT 20;
my table stucture
item
item id | category id
----------------------------
1 | 1
2 | 1
3 | 2
4 | 2
5 | 1
6 | 3
if my table has
category
category | count(item)
-----------------------------
1 | 5
2 | 3
3 | 10
4 | 8
5 | 12
6 | 6
7 | 6
8 | 6
I want to select 20 items like that
category items from this category
----------
1 | 4
2 | 3
3 | 4
4 | 4
5 | 4
6 | 1

Answer 1

You can try this query
SET #level = 0;
SET #group = '';
SELECT
item_id,
category_id
FROM (
SELECT
item_id,
category_id,
#level := IF(#group = category_id, #level+1, 1) AS level,
#group := category_id as EGroup
FROM item
ORDER BY category_id
) rs
WHERE level < 5
Here is some sample data
| ID | ITEM_ID | CATEGORY_ID |
|----|---------|-------------|
| 1 | 1 | 1 |
| 2 | 2 | 1 |
| 3 | 3 | 1 |
| 4 | 4 | 1 |
| 5 | 5 | 1 |
| 6 | 6 | 2 |
| 7 | 7 | 2 |
| 8 | 8 | 2 |
| 9 | 9 | 2 |
| 10 | 10 | 3 |
| 11 | 11 | 3 |
| 12 | 12 | 3 |
| 13 | 13 | 3 |
| 14 | 14 | 3 |
| 15 | 15 | 3 |
| 16 | 16 | 4 |
| 17 | 17 | 4 |
| 18 | 18 | 5 |
| 19 | 19 | 5 |
| 20 | 20 | 5 |
| 21 | 21 | 5 |
| 22 | 22 | 5 |
| 23 | 23 | 6 |
| 24 | 24 | 7 |
| 25 | 25 | 7 |
Output
| ITEM_ID | CATEGORY_ID |
|---------|-------------|
| 1 | 1 |
| 2 | 1 |
| 3 | 1 |
| 4 | 1 |
| 9 | 2 |
| 8 | 2 |
| 7 | 2 |
| 6 | 2 |
| 15 | 3 |
| 14 | 3 |
| 13 | 3 |
| 12 | 3 |
| 16 | 4 |
| 17 | 4 |
| 22 | 5 |
| 21 | 5 |
| 20 | 5 |
| 19 | 5 |
| 23 | 6 |
| 24 | 7 |
| 25 | 7 |
SQL Fiddle Demo