Mysql group by (custom number for result) - mysql

Question

I have a table
Id Name Id_collection Price
1 good1 2 10
2 good2 1 101
3 good3 3 102
4 good4 2 10
5 good5 2 10
I need to Group By id_collection, but i need to show 2 rows (to be able to change this value via variable or ... Ex: to change to 3 or 4 )
not
1
2
3
Ex
1
1
2
2
3
3
or
Ex
1
1
1
2
2
2
3
3
3
so the result must be
Id Name Id_collection Price
1 good1 2 10
4 good4 2 10
2 good2 1 101
3 good3 3 102
I was thinking about procedure or loop, but i didn't that before, Please help!!!

Answer 1

You want to group by adjacent values . . . in MySQL. You can use variables to assign the group. Alternatively, you can use this method to assign the group: count the number of rows that have id_collection different from each row with a smaller id.
You don't specify how to calculate the other columns, but here is a guess:
select min(id) as id, min(name) as name, id_collection, avg(price) as price
from (select t.*,
(select count(*)
from t t2
where t2.id_collection <> t.id_collection and
t2.id < t.id
) as grp
from t
) t
group by id_collection, grp;
EDIT:
I just realized that you probably don't want to aggregate the results; you probably just want the first row. For that, use variables:
select t.*
from (select t.*,
(#rn := if(#id = id_collection, #rn,
if(#id := id_collection, #rn + 1, #rn + 1)
)
) as rn
from t cross join
(select #id = -1, #rn := 0) params
order by id
) t
where rn = 1;

Answer 2

I edited a little the answer which #Gordon_Linoff posted, and it works now, you just have to change rn <= 3 this number and will get the various results.
#Gordon_Linoff - Thank you, this really helped me
select t.*
from (select t.*,
(#rn := if(#id = id_collection, #rn + 1,
if(#id := id_collection, 1, 0)
)
) as rn
from t cross join
(select #id := -1, #rn := 0) params
order by id_collection
) t
where rn <= 3;