How to add comment to a column in a table in Postgres?
To update comment, don't use alter command.

Here is the PostgreSQL syntax:

comment on column app_user_bookings.mod is '1# mobile booking,    2# admin booking,    3# web booking, 4# tell call'


It makes sense to add a check to prevent wrong values appearing:

ALTER TABLE app_user_bookings
ADD CHECK (mod IN (1,2,3,4));