Order by logic and get mixed results? - mysql

Question

I have this table structure
id | status
-----+------+
1 | a |
2 | b |
3 | b |
4 | b |
5 | a |
6 | a |
7 | b |
8 | a |
I have two statuses "a" and "b".
I need to order this by logic that says this: for each two "a", show one "b".
So something like this (the "a" is more important, so if there are a lot of "b"s they will just be left in the end)
id | status
-----+------+
1 | a |
5 | a |
2 | b |
6 | a |
8 | a |
3 | b |
4 | b |
7 | b |
Is there a way to do this?

Answer 1

select id
,status
from (select status
,id
,#prev_status := #status
,#status := status
,#rn := case when #prev_status = status
then #rn + 1
else 1
end as rn
from mytable t1,(select #status:=null,#rn:=0) x
order by status
,id
) t
order by floor((rn-1) / case status when 'a' then 2 else 1 end)
,case status when 'a' then 1 else 2 end
,rn
;
+----+--------+
| id | status |
+----+--------+
| 1 | a |
| 5 | a |
| 2 | b |
| 6 | a |
| 8 | a |
| 3 | b |
| 4 | b |
| 7 | b |
+----+--------+
This will help you to understand the solution:
(group_id = floor((rn-1) / case status when 'a' then 2 else 1 end))
+--------+----+----+----------+
| status | id | rn | group_id |
+--------+----+----+----------+
| a | 1 | 1 | 0 |
| a | 5 | 2 | 0 |
| a | 6 | 3 | 1 |
| a | 8 | 4 | 1 |
| b | 2 | 1 | 0 |
| b | 3 | 2 | 1 |
| b | 4 | 3 | 2 |
| b | 7 | 4 | 3 |
+--------+----+----+----------+

Answer 2

Use variables to enumerate the values and then some simple logic:
select id, status
from (select t.*,
(#rn := if(#s = status, #rn + 1,
if(#s := status, 1, 1)
)
) as rn
from t cross join
(select #rn := 0, #s := '') params
where status in ('a', 'b')
order by status, id
) ab
order by (case when status = a then floor( (rn - 1) / 2) else (rn - 1) end),
status, id;