Join with Group By & Order by using Case in MySQL - mysql

Question

Why this query wont work? Is it beacause combinaton of order by and group by?
One table is with adverts, other with subscriptions, third is with services, and fourth is many to many relation between services and locations (location is position where advert should be shown).
What i want is to order adverts stored in adverts table having location 2 first, then those who don't have location defined and then with location 1 (this order is generated programmicaly)
adverts table:
id, name, subscription_id
subscriptions table:
subscription_id, service_id, date, paid etc...
service_locations table:
service_id, location_id
as you can se there is fourth table in this case, but it is unimportant
The query:
select adverts.id, GROUP_CONCAT(service_locations.location_id) AS locations from adverts
left join subscriptions
on adverts.subscription_id = subscriptions.id
left join service_locations
on service_locations.service_id = subscriptions.service_id
group by adverts.id
order by case service_locations.location_id
when 2 then 1
when 1 then 3
else 2
end
Expected results:
+----+-----------+
| id | locations |
+----+-----------+
| 1 | 2 |
| 3 | 1,2 |
| 2 | null |
+----+-----------+
What i actually get (the third in row has location 2 but it is placed after null):
+----+-----------+
| id | locations |
+----+-----------+
| 1 | 2 |
| 2 | null |
| 3 | 1,2 |
+----+-----------+

Answer 1

When you use group by, all columns not in the group by should have aggregation functions. So, I think you intend something like this:
select a.id, GROUP_CONCAT(sl.location_id) AS locations
from adverts a left join
subscriptions s
on a.subscription_id = s.id left join
service_locations sl
on sl.service_id = s.service_id
group by a.id
order by max(case sl.location_id
when 2 then 1
when 1 then 3
else 2
end);
I'm not sure if max() is what you really need, but you do need an aggregation function. This specifically produces the output in the question:
order by (case min(sl.location_id)
when 2 then 1
when 1 then 2
else 3
end);

Answer 2

I have found a solution, order by must be executed before group by, which is not a default behaivor, more about that behaivour here: https://stackoverflow.com/a/14771322/4329156) (a subquery must be used)
So, query should look like
select *, GROUP_CONCAT(location_id) as locations from (
select adverts.id AS id, service_locations.location_id AS location_id from adverts
left join subscriptions
on adverts.subscription_id = subscriptions.id
left join service_locations
on service_locations.service_id = subscriptions.service_id
order by case service_locations.location_id
when 2 then 1
when 1 then 3
else 2
end
) as table
group by table.id
order by case table.location_id
when 2 then 1
when 1 then 3
else 2
end