MySQL每个类别的类别限制N

Question

TABLE

id       title        category

---

1        hello1          1
2        hello2          2
3        hello3          1

Query

select  *
from    videos
where   category in
        (
        select  category
        from    videos
        group by 
               category
        having 
               count(*) < 3
ORDER BY RAND()
        )

my goal is to get 2 titles from each category in a random order also I want to fetch records in this manner

category1
title1
title2

---

category2
title1
title2

Answer 1

The below query gives no more than two random rows from each category:

SELECT title, category
FROM (
  SELECT v.*,
     if( category = @last_cat,
         if( @last_cat:=category, @x:=@x+1,@x:=@x+1),
         if( @last_cat:=category, @x:=0,@x:=0)
     ) x
  FROM (SELECT @last_cat:=-9876, @x:=-91234) x,
       (SELECT * FROM videos ORDER BY category, rand()) v
) x
WHERE x < 2

demo: http://sqlfiddle.com/#!2/59cf9/8

Answer 2

UPDATED

Please try this updated query ( SQL Fiddle - http://sqlfiddle.com/#!2/de35bb/9 ):

select  videos.*
from    videos
where   
    (
        select  COUNT(vid.id)
        from    videos AS vid
        WHERE videos.category = vid.category
    ) <= 2
ORDER BY
  videos.category, RAND()

I've managed to find the solution on SO here: Using LIMIT within GROUP BY to get N results per group? in which answer points to the article How to select the first/least/max row per group in SQL where author describes a few techniques of performing such task. The above query was built upon an example provided in the second article, but there are other forms of solving this problem.