mysql select top N records conditionally - mysql

Question

i have the following table :
+------+------+-----+---------+------------+
| no | code |eot |group_id | compulsary |
+------+------+-----+---------+------------+
| 1005 | 101 | 51 | 1 | 1 |
| 1005 | 102 | 67 | 1 | 1 |
| 1005 | 121 | 65 | 1 | 1 |
| 1005 | 231 | 82 | 2 | 0 |
| 1005 | 232 | 56 | 2 | 0 |
| 1005 | 233 | 45 | 2 | 1 |
| 1005 | 313 | 80 | 3 | 0 |
| 1005 | 443 | 50 | 4 | 0 |
|------+------+-----+---------+------------+
now what i want is :
1.) return all records where group_id=1,
2.) return the best two from group_id=2(however if compulsary=1 then include that row and the best from the remaining group_id=2),
3.)return 1 row each from group_id=3 and group_id=4 and if compulsary=1 then return that row
the final result should have only seven rows:
+------+------+-----+---------+------------+
| no | code |eot |group_id | compulsary |
+------+------+-----+---------+------------+
| 1005 | 101 | 51 | 1 | 1 |
| 1005 | 102 | 67 | 1 | 1 |
| 1005 | 121 | 65 | 1 | 1 |
| 1005 | 231 | 82 | 2 | 0 |
| 1005 | 233 | 45 | 2 | 1 |
| 1005 | 313 | 80 | 3 | 0 |
| 1005 | 443 | 50 | 4 | 0 |
|------+------+-----+---------+------------+
with the compulsary=1 rows inluded like above;
so far I have this query though I don't know how to check for the compulsary to get what I want:
select rg.*
from
(
select *
from
(
select
rgrade.*,
#rn := if(#gr=group_id,if(#gr:=group_id,#rn+1,#rn+1),if(#gr:=group_id,1,1)) as rn
from rgrade
cross join (select #rn:=0,#gr:=0) as vars
where admission_no=1005
) v
where (group_id=1)
or (group_id=2 and if(compulsary=1,rn<=1,rn<=2))
or (group_id in (3,4) and rn=1)
) rg
order by group_id;
the query returns the seven rows as expected but does not check for compulsary in group_id=2.
Any help much appreciated

Answer 1

You are describing several queries here, the result of which you want combined: All group_id = 1, the two best group_id = 2, the best of group_id = 3, and the best of group_id = 4. So write these queries and combine them with UNION ALL. With "best" defined as compulsary = 1 preferred, then highest eot, you get:
(select * from mytable where group_id = 1)
union all
(select * from mytable where group_id = 2 order by compulsary = 1 desc, eot desc limit 2)
union all
(select * from mytable where group_id = 3 order by compulsary = 1 desc, eot desc limit 1)
union all
(select * from mytable where group_id = 4 order by compulsary = 1 desc, eot desc limit 1)
order by group_id, no, code
;

Answer 2

You are trying to mimic standard SQL's
row_number() over (partition by group_id
order by case when compulsary = 1 then 1 else 0 end desc, eot desc)
with MySQL means. And I see the partitioning by group_id in your query, but I don't see any ORDER BY to get the best records first.
Here is my attempt on it. There may be mistakes; I'm no MySQL guy.
select
no, code, eot, group_id, compulsary
from
(
select
no,
code,
eot,
compulsary,
#row_number :=
case when group_id = #last_group_id then #row_number + 1 else 1 end as row_number,
#last_group_id := group_id as group_id
from rgrade
cross join
(
select
#row_number := 0,
#last_group_id := -1
) as vars
where admission_no = 1005
order by group_id, (compulsary = 1) desc, eot desc
) ranked
where (group_id = 1)
or (group_id = 2 and row_number <= 2)
or (group_id = 3 and row_number = 1)
or (group_id = 4 and row_number = 1)
order by group_id, code;