Getting the avg of the top 10 students from each school
We have a school district with 38 elementary schools. The kids took a test. The averages for the schools are widely dispersed, but I want to compare the averages of JUST THE TOP 10 students from each school.
Requirement: use temporary tables only.
I have done this in a very work-intensive, error-prone sort of way as follows.
(sch_code = e.g., 9043; -- schabbrev = e.g., "Car开发者_如何转开发ter"; -- totpct_stu = e.g., 61.3)DROP TEMPORARY TABLE IF EXISTS avg_top10 ;
CREATE TEMPORARY TABLE avg_top10
( sch_code VARCHAR(4),
schabbrev VARCHAR(75),
totpct_stu DECIMAL(5,1)
);
INSERT
INTO avg_top10
SELECT sch_code
, schabbrev
, totpct_stu
FROM test_table
WHERE sch_code IN ('5489')
ORDER
BY totpct_stu DESC
LIMIT 10;
-- I do that last query for EVERY school, so the total
-- length of the code is well in excess of 300 lines.
-- Then, finally...
SELECT schabbrev, ROUND( AVG( totpct_stu ), 1 ) AS top10
FROM avg_top10
GROUP
BY schabbrev
ORDER
BY top10 ;
-- OUTPUT:
-----------------------------------
schabbrev avg_top10
---------- ---------
Goulding 75.4
Garth 77.7
Sperhead 81.4
Oak_P 83.7
Spring 84.9
-- etc...
Question: So this works, but isn't there a lot better way to do it?
Thanks!
PS -- Looks like homework, but this is, well...real.
Using this technique.
select sch_code,
schabbrev,
ROUND( AVG( totpct_stu ), 1 ) AS top10
from (select sch_code,
schabbrev,
totpct_stu,
@num := if(@group = sch_code, @num + 1, 1) as row_number,
@group := sch_code as dummy
from test_table
order by sch_code, totpct_stu desc) as x
where row_number <= 10
GROUP BY sch_code,
schabbrev
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