Concatenate non empty CONCAT_GROUP to parent column in MySQL - mysql

Question

I need to get the list of categories from store_cat, with the child COUNT from store_item (amount of products) and GROUP_CONCAT from store_cat_attributes (list of attributes). The thing is, using CONCAT function I need to attach the GROUP_CONCAT value with name column in the parent table (store_cat), and that's where it gets tricky.
This works fine:
SELECT
store_cat.id_cat AS id,
store_cat.name AS name,
GROUP_CONCAT(store_cat_attribute.name SEPARATOR ", ") AS attributes,
COUNT(store_item.id_item) AS products,
store_cat.flg_public AS flg_public
FROM store_cat
LEFT JOIN store_item ON store_item.id_cat = store_cat.id_cat
LEFT JOIN store_cat_attribute ON store_cat_attribute.id_cat = store_cat.id_cat
WHERE store_cat.id_store = 1
GROUP BY store_cat.id_cat
ORDER BY name
But this is what I would actually need. The problem is that, when I execute this query, the store_cat.name value shows an empty value when there are no attributes:
SELECT
store_cat.id_cat AS id,
CONCAT(store_cat.name, " (", GROUP_CONCAT(store_cat_attribute.name SEPARATOR ", "), ")") AS name,
COUNT(store_item.id_item) AS products,
store_cat.flg_public AS flg_public
FROM store_cat
LEFT JOIN store_item ON store_item.id_cat = store_cat.id_cat
LEFT JOIN store_cat_attribute ON store_cat_attribute.id_cat = store_cat.id_cat
WHERE store_cat.id_store = 1
GROUP BY store_cat.id_cat ORDER BY name
Basically, the idea is that the store_cat.name column should contain the attributes list with CONCAT and GROUP_CONCAT, just like this:
Comidas
Correas (S, M, L, XL)
Juguetes
Medicinas
Here's the current SQLfiddle. By the way, something is off with the attributes order in the current GROUP_CONCAT. It is displaying (XL, S, M, L) instead of (S, M, L, XL).
Problems to solve:
Use GROUP_CONCAT to concatenate the attributes to the category name only when there are attributes.
Use the store_cat_attributes.position to set the order for the GROUP_CONCAT values.
Any ideas? Thanks!

Answer 1

The following expression should return the results that you expect :
CONCAT(
store_cat.name,
IFNULL(
CONCAT(
' (',
GROUP_CONCAT(
store_cat_attribute.name
ORDER BY store_cat_attribute.position
SEPARATOR ', '
),
')'
),
''
)
) AS name
Basically, this just tries to GROUP_CONCAT() the attributes, and if the result is NULL then it turns the attribute list to an empty string. Please note that GROUP_CONCAT support ORDER BY.
I also fixed the GROUP BY clause : in non-ancient versions of MySQL, all non-aggregared columns must appear in the where clause (you are missing store_cat.name).
Demo on DB Fiddle with your sample data :
SELECT
store_cat.id_cat AS id,
CONCAT(
store_cat.name,
IFNULL(
CONCAT(
' (',
GROUP_CONCAT(store_cat_attribute.name ORDER BY store_cat_attribute.position SEPARATOR ', '),
')'
),
''
)
) AS name,
COUNT(store_item.id_item) AS products,
store_cat.flg_public AS flg_public
FROM
store_cat
LEFT JOIN store_item ON store_item.id_cat = store_cat.id_cat
LEFT JOIN store_cat_attribute ON store_cat_attribute.id_cat = store_cat.id_cat
WHERE store_cat.id_store = 1
GROUP BY store_cat.id_cat, store_cat.name
ORDER BY name;
| id | flg_public | name | products |
| --- | ---------- | --------------------- | -------- |
| 3 | 1 | Comidas | 0 |
| 2 | 1 | Correas (S, M, L, XL) | 4 |
| 1 | 1 | Juguetes | 2 |
| 4 | | Medicinas | 0 |