Question

I’ve been using GROUP BY for all types of aggregate queries over the years. Recently, I’ve been reverse-engineering some code that uses PARTITION BY to perform aggregations.

In reading through all the documentation I can find about PARTITION BY, it sounds a lot like GROUP BY, maybe with a little extra functionality added in.

Are they two versions of the same general functionality or are they something different entirely?

Answer 1

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

• 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.