What's the significance of having null as a column ouput all nulls in SQL?

[vinayak jog]

Simple query

select column_name, null from table_name;

what is the use of null here ?

[Matthew Brown]

It's going to put a column in the output containing all nulls.

Pretty pointless by itself, but there may be cases where it's useful in conjunction with some other constructions. For example:
INSERT INTO ....
SELECT column_name, null
...