Mixing different categories results, ordered by score in MySQL

Question

In my PHP application, I have a mysql table of articles which has the following columns:

article_id    articletext    category_id    score

Answer 1

Just for learning purpose. I made a test with 3 categories. I have no idea how this query could run on a large recordset.

select * from (
(select @r:=@r+1 as rownum,article_id,articletext,category_id,score
from articles,(select @r:=0) as r
where category_id = 1
order by score desc limit 100000000) 
union all
(select @r1:=@r1+1,article_id,articletext,category_id,score
from articles,(select @r1:=0) as r
where category_id = 2
order by score desc limit 100000000)
union all
(select @r2:=@r2+1,article_id,articletext,category_id,score
from articles,(select @r2:=0) as r
where category_id = 3
order by score desc limit 100000000)
) as t
order by rownum,score desc