They‘re used in different places. group by
modifies the entire query, like:
select customerId, count(*) as orderCount
from Orders
group by customerId
But partition by
just works on a window function, like row_number
:
select row_number() over (partition by customerId order by orderId)
as OrderNumberForThisCustomer
from Orders
A group by
normally reduces the number of rows returned by rolling them up and calculating averages or sums for each row. partition by
does not affect the number of rows returned, but it changes how a window function‘s result is calculated.