I have a table categories containing categories id having the records of 1 to 20.

when i am doing the search query by using the IN function in mysql i got the results. but i am struggling to get the results using order by.

The limit may be vary for every search as this is one of the input value.

For example I have tried this query to find out the search results,

select *  from categories where category in (20,16,12,8)   order by rand(), id limit 0,6

this query is executed and the results are in random category_id. the results will be,

8
12
16
20

and following this, another two records must be

8
12

If category_id contains only one record for 8 then, it should follow from 12,16.

How can i achieve this ? thanks in advance.