Compute 2,3 quartile average in SQL

Question

I want to write a stored proc in SQL (MySQL) to compute the average of second and third quartiles.

In other words I have records for measurements for how long it takes for an URL to load. Records are (id,url,time) and they are many measurements for each URL. What I'm trying to do is for each URL remove the lowest and top 25% (i.e. lower and upper quartiles) and compute average of the remaining 25%-75% of loading times. And store this into another table.

I saw some examples for this for MS SQL and seemed to be relatively easy. But I have to use MySQL where :

• LIMIT clause doesn't support percents ( no analogue to select top 25% )
• LIMIT clause doesn't support its arguments to be variables (only constants)
• functions don't support dynamic SQL (e.g. PREPARE and EXECUTE )

And I got as far as here :

create procedure G(
  IN val VARCHAR(10)
)
Begin
  select @cnt:=count(*) from test where a=val;
  select  @of:= @cnt /4;
  SELECT @len:= @cnt/2; 
  Prepare stmt from 'select * from test where a="a" LIMIT ?,?';
  execute stmt using @of, @len;
END;

I can write it in PHP but think in SQL it would have much better overall performance. I will appreciate some help very much.

Answer 1

Look at answer and comment by @Richard aka cyberkiwi in this question:

Select *
from
(
    SELECT tbl.*, @counter := @counter +1 counter
    FROM (select @counter:=0) initvar, tbl
    ORDER BY ordcolumn
) X
where counter >= (25/100 * @counter) and counter <= (75/100 * @counter);
ORDER BY ordcolumn