Aggregate SQL Function to grab only the first from each group

Question

I have 2 tables - an Account table and a Users table. Each account can have multiple users. I have a scenario where I want to execute a single query/join against these two tables, but I want all the Account data (Account.*) and only the first set of user data (specifically their name).

Instead of doing a "min" or "max" on my aggregated group, I wanted to do a "first". But, apparently, there is no "First" aggregate function in TSQL.

Any suggestions on how to go about getting this query? Obviously, it is easy to get the cartesian product of Account x Users:

 SELECT User.Name, Account.* FROM Account, User
 WHERE Account.ID = User.Account_ID

But how might I got about only getting the first user from the product based on the order of their User.ID ?

Answer 1

Rather than grouping, go about it like this...

select
    *

from account a

join (
    select 
        account_id, 
        row_number() over (order by account_id, id) - 
            rank() over (order by account_id) as row_num from user
     ) first on first.account_id = a.id and first.row_num = 0