select distinct values on 1 column only in oracle sql developer

Question

i have 3 columns.

column_a, column_b, column_c

I am trying to get all the rows with making distinct only in column_a.

when i write

select distinct column_a,column_b,column_c

i think it gives me distinct pairs. So i have

value1 - a - b
value1 - a - c

I want to keep the distinct values of column_a following by column_b and column_c values cause i will create a table from that sql query and i want to add PK the column_a column.

Answer 1

I think the safest way is to use row_number() :

select a, b, c
from (select t.*,
             row_number() over (partition by a order by a) as seqnum
      from t
     ) t
where seqnum = 1;

This will return an arbitrary row for each a value, but a will not be repeated in the result set.