How to generate row numbers in teradata?

We have come across rownum function in oracle . This function gives number for every row/record in Oracle. In teradata there is no direct approach like in oracle. There are various approaches suggested around.

Approach 1:
Here Sum function is used  over rows preceding in the SOURCE_TABLE

select
sum(1) over( rows unbounded preceding ),
columnA,
columnB
from
SOURCE_TABLE;

Approach 2:
Here ROW_NUMBER function is used to generate row_number on columnA
select
ROW_NUMBER() over( ORDER BY columnA ),
columnA,
columnB
from
SOURCE_TABLE;


If you have  to use the  row number concept  in target table  as well, then  following  approach using "identity column" (from V2R6 onwards )  be used :

CREATE MULTISET TABLE TARGET_TABLE
  (
   columnA INTEGER GENERATED BY DEFAULT AS IDENTITY
       (START WITH 1
        INCREMENT BY 20) ,
   columnB VARCHAR(20) NOT NULL
  )
UNIQUE PRIMARY INDEX pidx (ColA);

P.S:  Identity columns does differ from sequence concept in oracle. The numbers assigned in these columns are not guaranteed to be sequenctial. The Identity column in Teradata is used to guaranteed row-uniqueness.

This  works without use of Identity approach.
create TABLE TARGET_TABLE as
(
   select
   ROW_NUMBER() over( ORDER BY columnA ) NUMBER_SEQ ,
   columnA,
   columnB,
   columnC
   from a join b on a.id=b.id
) with data ;