Limiting rows in SQL doesn't work properly - sql

Question

I want to get the first 5 rows of every season in my select. I have 4 seasons: SUM, SPR, AUT, WIN.
So there should be 20 rows in total.
My select looks like this:
select *
from (
select year, season, ROUND(avg(temperature),1) as avgTemp
from temperature join month on temperature.MONTH = month.MONTH
group by (season, year)
order by season, avgTemp asc
) where rownum <= 5;
It works for just one season. The output is:
1993 AUT 8,7
2007 AUT 9,9
1996 AUT 10
1998 AUT 10
2008 AUT 10,5
But it should look like that:
1996 SPR 9.6
1991 SPR 10.3
2006 SPR 10.3
2004 SPR 10.6
1995 SPR 10.6
1996 SUM 18.9
1993 SUM 19.1
2007 SUM 19.5
1998 SUM 19.5
2000 SUM 19.6
1993 AUT 8.7
2007 AUT 9.9
1998 AUT 10.0
1996 AUT 10.0
2008 AUT 10.5
1996 WIN .3
1991 WIN 1.2
2003 WIN 1.6
2006 WIN 1.9
2005 WIN 2.0
Do you know how to improve the select or do you have any other suggestions? Thanks in advance!

Answer 1

You need to do it in three steps:
Group by season and year, calculating the average temperature
Assign a row number: it restart with each season and assigns in ascending order according to the average temperature
Select only the rows with a row number between 1 and 5
The SQL should look like this (untested):
select year, season, avg_temp
from (
select year, season, avg_temp,
row_number() over(partition by season order by avg_temp) rn
from (
select year, season, ROUND(avg(temperature),1) as avg_temp
from temperature
join month on temperature.MONTH = month.MONTH
group by season, year
)
)
where rn <= 5;
Update
For you special ordering by season, add this:
order by case season
when 'SPR' then 1
when 'SUM' then 2
when 'AUT' then 3
when 'WIN' then 4
end, avg_temp;

Answer 2

WITH cteAverageTempByYearBySeason AS (
SELECT
year
,season
,ROUND(AVG(temperature),1) as AvgTemp
FROM
Temperature t
INNER JOIN Month m
On t.MONTH = m.MONTH
GROUP BY
year
,season
)
, cteRowNumber AS (
SELECT
*
,ROW_NUMBER() OVER (PARTITION BY season ORDER BY AvgTemp ASC) as RowNumber
FROM
cteAverageTempByYearBySeason
)
SELECT *
FROM
cteRowNumber
WHERE
RowNumber <= 5
Here is an example. I broke out the derived tables into Common Table Expressions to make the logic more noticeable. You need to create a PARTITIONED ROW_NUMBER() not just use oracles special rownumber. The latter will only return the same as TOP/LIMIT 5 where as the former will allow you to identify 5 rows per season.
Edit added a neat trick for your order by so you don't have to write a case expression. This one utilizes your month number which I assume is what MONTH column is.
WITH cteAverageTempByYearBySeason AS (
SELECT
year
,season
,ROUND(AVG(temperature),1) as AvgTemp
,MAX(m.MONTH) as SeasonOrderBy
FROM
Temperature t
INNER JOIN Month m
On t.MONTH = m.MONTH
GROUP BY
year
,season
)
, cteRowNumber AS (
SELECT
*
,ROW_NUMBER() OVER (PARTITION BY season ORDER BY AvgTemp ASC) as RowNumber
FROM
cteAverageTempByYearBySeason
)
SELECT
year
,season
,AVG
FROM
cteRowNumber
WHERE
RowNumber <= 5
ORDER BY
SeasonOrderBy
,AvgTemp
,Year

Answer 3

You need to use row_number to get 5 rows for each grouping:
select
year,
season,
round(avg(temperature), 1) as avgTemp
from (
select *,
row_number() over(partition by season, year order by season, avgTemp) as rn
from temperature t
join month m
on m.MONTH = t.MONTH
) a
where
a.rn <= 1