How to user PARTITION BY without GROUP BY clause

Question

I have a simple query where I need to sum up all Premium per policy Number. It gives me an error because I need to group it by Premium. Is any way to achieve this without grouping by "Premium"?

Sample query:

declare @Test table (PolicyNumber varchar(50), Coverage varchar(50), Premium money)
insert into @Test 
select 'Pol1' as PolicyNumber, 'Coverage1' as Coverage, 100 as Premium
UNION ALL select 'Pol1', 'Coverage2', 200 
UNION ALL select 'Pol1', 'Coverage2', 25 
UNION ALL select 'Pol1', 'Coverage3', 500 
UNION ALL select 'Pol1', 'Coverage4', 300 
UNION ALL select 'Pol1', 'Coverage4', 25 
UNION ALL select 'Pol1', 'Coverage5', 150 


select 
PolicyNumber,
Coverage,
SUM(Premium) as Premium,
-- this gives an error
PremiumPerPolicy = SUM(Premium) OVER (Partition by PolicyNumber)
from @Test
group by PolicyNumber, Coverage

The result should look like this

PolicyNumber    Coverage    Premium  PremiumPerPolicy
 Pol1          Coverage1     100       1300
 Pol1          Coverage2     225       1300
 Pol1          Coverage3     500       1300
 Pol1          Coverage4     325       1300
 Pol1          Coverage5     150       1300

Answer 1

You can aggregate your value normally and then aggerate again using a window function.

SUM(Premium) OVER (Partition by PolicyNumber)

becomes

SUM(SUM(Premium)) OVER (Partition by PolicyNumber)

Answer 2

Just pre-calculate your partition sum first. I have also assumed that its possible to have multiple policy numbers (again not shown in your sample data).

with cte as (
    select
        PolicyNumber
        , Coverage
        , Premium
        , sum(Premium) over (partition by PolicyNumber) PremiumSum
    from @Test
)
select PolicyNumber, Coverage, sum(premium), PremiumSum
from cte
group by PolicyNumber, Coverage, PremiumSum;