Question

I have a table Table like this:

ColumnA (VARCHAR)|ColumnB (VARCHAR)|ColumnC (VARCHAR)|ColumnD (DATE)
     A1                 B1               C1          2012-12-06
     A1                 B1               C1          2012-12-12
     A2                 B2               C2          2012-12-07
     A2                 B2               C2          2012-12-09

I want to select only the 1st occurrence combined to ColumnA-ColumnB-ColumnC so the result will be something like this:

A1 | B1 | C1 | 2012-12-06
A2 | B2 | C2 | 2012-12-07

Answer 1

You can use an aggregate function on the Date column and the group by the rest:

select ColumnA, ColumnB, ColumnC, Min(ColumnD)
from yourtable
group by ColumnA, ColumnB, ColumnC

See SQL Fiddle with Demo

Or you can use row_number():

select ColumnA, ColumnB, ColumnC, ColumnD
from
(
  select ColumnA, ColumnB, ColumnC, ColumnD,
    row_number() over(partition by ColumnA, ColumnB, ColumnC order by ColumnD) rn
  from yourtable
) src
where rn = 1

See SQL Fiddle with Demo