How do I (or can I) SELECT DISTINCT on multiple columns?

Question

I need to retrieve all rows from a table where 2 columns combined are all different. So I want all the sales that do not have any other sales that happened on the same day f

Answer 1

If your DBMS doesn't support distinct with multiple columns like this:

select distinct(col1, col2) from table

Multi select in general can be executed safely as follows:

select distinct * from (select col1, col2 from table ) as x

As this can work on most of the DBMS and this is expected to be faster than group by solution as you are avoiding the grouping functionality.