Question

I have a table that has data like below(sorted by date)

COL_A | COL_B

Item1 | Date1
Item2 | Date1
Item2 | Date2
Item1 | Date3
Item3 | Date3

Now I want a query that will return the first instance of every Item ie the result set should be like;

COL_A | COL_B

Item1 | Date1
Item2 | Date1
Item3 | Date3

How to do this?

Answer 1

If all you need is the first instance of each and no further information then its a simple GROUP BY with MIN:

select Col_A, min(Col_B) MinColB
from tbl
group by Col_A

If you want further information from the table having found the first instance:

select t1.Col_A, t1.Col_B, t1.otherField...
from tbl t1
  inner join (select Col_A, min(Col_B) MinColB
              from tbl
              group by Col_A) t2 on t2.Col_A = t1.Col_A and t2.MinColB = t1.Col_B

You use the derived table to find the first date per Col_A and then join back to the table to retrieve further details