Group by after a partition by in MS SQL Server - sql

Question

I am working on some car accident data and am stuck on how to get the data in the form I want.
select
sex_of_driver,
accident_severity,
count(accident_severity) over (partition by sex_of_driver, accident_severity)
from
SQL.dbo.accident as accident
inner join SQL.dbo.vehicle as vehicle on
accident.accident_index = vehicle.accident_index
This is my code, which counts the accidents had per each sex for each severity. I know I can do this with group by but I wanted to use a partition by in order to work out % too.
However I get a very large table (I assume for each row that is each sex/severity. When I do the following:
select
sex_of_driver,
accident_severity,
count(accident_severity) over (partition by sex_of_driver, accident_severity)
from
SQL.dbo.accident as accident
inner join SQL.dbo.vehicle as vehicle on
accident.accident_index = vehicle.accident_index
group by
sex_of_driver,
accident_severity
I get this:
sex_of_driver
accident_severity
(No column name)
1
1
1
1
2
1
-1
2
1
-1
1
1
1
3
1
I won't give you the whole table, but basically, the group by has caused the count to just be 1.
I can't figure out why group by isn't working. Is this an MS SQL-Server thing?
I want to get the same result as below (obv without the CASE etc)
select
accident.accident_severity,
count(accident.accident_severity) as num_accidents,
vehicle.sex_of_driver,
CASE vehicle.sex_of_driver WHEN '1' THEN 'Male' WHEN '2' THEN 'Female' end as sex_col,
CASE accident.accident_severity WHEN '1' THEN 'Fatal' WHEN '2' THEN 'Serious' WHEN '3' THEN 'Slight' end as serious_col
from
SQL.dbo.accident as accident
inner join SQL.dbo.vehicle as vehicle on
accident.accident_index = vehicle.accident_index
where
sex_of_driver != 3
and
sex_of_driver != -1
group by
accident.accident_severity,
vehicle.sex_of_driver
order by
accident.accident_severity

Answer 1

You seem to have a misunderstanding here.
GROUP BY will reduce your rows to a single row per grouping (ie per pair of sex_of_driver, accident_severity values. Any normal aggregates you use with this, such as COUNT(*), will return the aggregate value within that group.
Whereas OVER gives you a windowed aggregated, and means you are calculating it after reducing your rows. Therefore when you write count(accident_severity) over (partition by sex_of_driver, accident_severity) the aggregate only receives a single row in each partition, because the rows have already been reduced.
You say "I know I can do this with group by but I wanted to use a partition by in order to work out % too." but you are misunderstanding how to do that. You don't need PARTITION BY to work out percentage. All you need to calculate a percentage over the whole resultset is COUNT(*) * 1.0 / SUM(COUNT(*)) OVER (), in other words a windowed aggregate over a normal aggregate.
Note also that count(accident_severity) does not give you the number of distinct accident_severity values, it gives you the number of non-null values, which is probably not what you intend. You also have a very strange join predicate, you probably want something like a.vehicle_id = v.vehicle_id
So you want something like this:
select
sex_of_driver,
accident_severity,
count(*) as Count,
count(*) * 1.0 /
sum(count(*)) over (partition by sex_of_driver) as PercentOfSex
count(*) * 1.0 /
sum(count(*)) over () as PercentOfTotal
from
dbo.accident as accident a
inner join dbo.vehicle as v on
a.vehicle_id = v.vehicle_id
group by
sex_of_driver,
accident_severity;