How to select main item row from table using CASE WHERE conditions? - mysql

Question

I have an array of products and I need to select the main photo of each product. My photo table consists of columns: id, product_id, photoname, flag, order.
'flag' = 1 when is the main photo and 'order' is the ordering of photos. Sometimes the 'flag' and 'order'are 0 (not set), so I need to assume the main photo is its 'id' ASC.
In my MySQL code I have:
private function getProducts($idUser) {
$products = DB::connection('mysql2')->select("SELECT
a.*
, (CASE WHEN photos.flag = 0 THEN
(
SELECT photoname
FROM mpy_product_photos
WHERE flag = 1 AND product_id = a.id LIMIT 1
)
ELSE photos.photoname END
) AS photoname
FROM mpy_products AS `a`
LEFT JOIN mpy_product_photos AS `photos` ON (photos.product_id = a.id)
WHERE a.user_id = '$idUsuario'
GROUP BY a.id
ORDER BY a.date DESC");
return $products;
}
When 'flag' = 0 for all 'photoname' from the same 'product_id', the return is null.
I've tried adding the following, along with many things but always get a Syntax error.
, (CASE WHEN photos.flag = 0 THEN
(
SELECT photoname
FROM mpy_product_photos
WHERE flag = 1 AND product_id = a.id LIMIT 1
)
ELSE
(
SELECT photoname
FROM mpy_product_photos
WHERE product_id = a.id ORDER BY photos.id LIMIT 1 END
)
) AS photoname
So when I don't have 'flag' = 1, I need to select and order by 'order' ASC. If 'order' = 0 in all rows, I need to select by 'id' ASC.
I've also tried the following, and at least I get a 'photoname' randomly (I guess) selected instead of null when all 'flag' = 0.
, (CASE WHEN photos.flag = 0 THEN
(
SELECT photoname
FROM mpy_product_photos
WHERE product_id = a.id ORDER BY photos.flag DESC LIMIT 1
)
ELSE photos.photoname END
) AS photoname
How can I solve this?

Answer 1

UPDATE: You specifically ask for a solution with CASE WHEN. In the meantime, I provide alternatives.
For MySQL 5.5+
select
a.*,
(select
photoname
from my_product_photos
where product_id = a.id
order by flag desc, ord asc, id asc
limit 1) as photoname
from my_product a;
If you're using MySQL 8.0, you can achieve this with the RANK window function rather simply as well
SELECT
product.*,
photo.photoname
FROM my_product product
JOIN (
SELECT
*,
RANK() OVER(PARTITION BY product_id ORDER BY flag DESC, ord ASC, id ASC) AS r
FROM my_product_photos
) photo ON photo.product_id = product.id AND photo.r = 1;
The RANK Function, as it's name implies, ranks the rows according to the given "rules". In the query, the rules are: Rank those with the higher flag, the lower order and lower id of the same product_id. This is going to be as a column of alias r. Then, you can just INNER JOIN as usual. With r you can then filter out the ones "less likely to be what you want".
This is a DB Fiddle with sample data. This might not match what you have in your own DB, so adjust as necessary.