Você pode usar Row_Number().
select UserId, Date, StoreId from (select row_number() over(partition
by UserId order by date) as RowNumber, UserId, Date, StoreId from
Transactions ) as View1 where RowNumber = 1
http://sqlfiddle.com/#!6/e536a/7
select UserId, Date, StoreId from (select row_number() over(partition
by UserId order by date) as RowNumber, UserId, Date, StoreId from
Transactions ) as View1 where RowNumber = 1
