T-SQL select rows by oldest date and unique category

Question

I\'m using Microsoft SQL. I have a table that contains information stored by two different categories and a date. For example:

ID   Cat1   Cat2   Date/Time         


        

Answer 1

Have a look at row_number() on MSDN.

SELECT  *
FROM    (
        SELECT  *,
                ROW_NUMBER() OVER (PARTITION BY col1, col2 ORDER BY date_time, id) rn
        FROM    mytable
        ) q
WHERE   rn = 1

(run the code on SQL Fiddle)