Question

Well, I have three different tables (check below). I wanted to make that I can see, which is the latest image (req_image_1, etc) saved for each category, which is not parent (cat_parent = 0)

One table, which holds general information about requests

+----+--------------------------+------------+
| id | req_name                 | req_parent |
+----+--------------------------+------------+
|  3 | Send pack                |         19 |
|  4 | Go Visit                 |         18 |
|  5 | Stop by                  |         19 |
|  6 | Deliver cookies          |         34 |
+----+--------------------------+------------+

Second table, which holds meta information about requests

+----------+------------+------------+----------------------------+
| umeta_id | request_id | meta_key   | meta_value                 |
+----------+------------+------------+----------------------------+
|       1  |          3 | req_city   | London                     |
|       2  |          3 | req_street | 11 Baker street            |
|       3  |          3 | req_img_1  | a1c8f69edb37bf6c6.jpg      |
|       4  |          4 | req_city   | Manchester                 |
|       5  |          4 | req_street | 71 Main street             |
|       6  |          4 | req_img_2  | a71f4160d7f7f7555.jpg      |
|       7  |          5 | req_city   | Sheffield                  |
|       8  |          5 | req_street | 240 Duke street            |
|       9  |          6 | req_city   | Manchester                 |
|       10 |          6 | req_street | 13 Chapel street           |
|       11 |          6 | req_img_1  | 854b9faaa53d8fe02.jpg      |
+----------+------------+------------+----------------------------+

Third table, which holds information about categories

+----+------------------------+------------+
| ID | cat_name               | cat_parent |
+----+------------------------+------------+
|  1 | Category_01            |          0 |
|  6 | Category_02            |          0 |
| 18 | Category_01_01         |          1 |
| 19 | Category_01_02         |          1 |
| 34 | Category_02_01         |          6 |
+----+------------------------+------------+

So far I managed, that I could get all images for each category with this query:

SELECT cat.cat_parent AS category, req.ID, meta.meta_value AS image
FROM d_requests req
LEFT JOIN d_requests_meta meta ON ( req.ID = meta.request_id ) 
LEFT JOIN d_categories cat ON ( req.req_parent = cat.ID ) 
WHERE meta.meta_key LIKE 'req_img_%'

I got this result:

+------------+----+-----------------------+
| category   | ID | image                 |
+------------+----+-----------------------+
|          1 |  3 | a1c8f69edb37bf6c6.jpg |
|          1 |  4 | a71f4160d7f7f7555.jpg |
|          6 |  6 | 854b9faaa53d8fe02.jpg |
+------------+----+-----------------------+

But I wanted to make enhancement, so I would get only result, where each category has only one image against, for example category 1, has image a71f4160d7f7f7555.jpg and category 6 has image 854b9faaa53d8fe02.jpg

I bet, that I miss some basic knowledge, and simple enhancement with subquery and selecting MAX would work as a charm.

Thanks!

Answer 1

SELECT category, ID, image
FROM (  SELECT cat.cat_parent AS category, req.ID, meta.meta_value AS image
        FROM d_requests AS req
        LEFT JOIN d_requests_meta AS meta
        ON req.ID = meta.request_id
        LEFT JOIN d_categories AS cat
        ON req.req_parent = cat.ID
        WHERE meta.meta_key LIKE 'req_img_%'
        ORDER BY req.ID DESC) AS h
GROUP BY category

I edited Clodoaldo’s answer with the use of the inofficial MySQL assumption that GROUP BY will return the 1st row based on ORDER BY in subquery.