Oracle db. How to get a result set of unique values of pretty much arbitrary amount of columns of the same table

Question

Having this column structure

Id, col_1, col_2, col_3
1    A      A      B
2    B      B      B
3    C      C      D

Is it possible to get one-column output with unique values?

res_column
 A
 B
 C
 D

What I've already tried:

1. Union seems doable only in case there are 2 to 3 columns, which is not the case.
2. I've found a pivot/unpivot, but I didn't really grasp it.

Answer 1

with
  sample_inputs (id, col_1, col_2, col_3) as (
    select 1, 'A', 'A', 'B' from dual union all
    select 2, 'B', 'B', 'B' from dual union all
    select 3, 'C', 'C', 'D' from dual
  )
select  distinct res_column
from    sample_inputs
unpivot (res_column for col in (col_1, col_2, col_3))
order   by res_column    ---   if needed
;

RES_COLUMN
----------
A
B
C
D

If you must handle null also, you can do so with the include nulls option to unpivot . Without this option, the query above will still work - it will just ignore (discard, exclude) all null from the unpivoted columns.

with
  sample_inputs (id, col_1, col_2, col_3) as (
    select 1, 'A' , 'A' , 'B'  from dual union all
    select 2, 'B' , 'B' , 'B'  from dual union all
    select 3, 'C' , 'C' , 'D'  from dual union all
    select 4, null, null, null from dual
  )
select  distinct res_column
from    sample_inputs
unpivot include nulls (res_column for col in (col_1, col_2, col_3))
order   by res_column    ---   if needed
;


RES_COLUMN
----------
A
B
C
D
 
 
(five rows selected)

Note that the last result has five rows, not four; the last value is null , which is invisible, but if you run the query in an editor that shows you the number of rows (or if you use an option to show null as <null> or such), then you will see it.

NOTE: You can't make this "generic" in plain SQL; the number of columns to unpivot, and their names, must be hard-coded in the query. If you need a "generic" solution you will need to use dynamic SQL; that is a much more advanced topic, and certainly not appropriate if you don't even grasp static unpivot yet.