How can i change existing column as Identity in PostgreSQL 11.1

Question

I would like to changes my existing column as Auto Identity in a Postgres Database.
I tried with below script but it won't worked.
Let me know if you have solution for the same
I don't want to use postgres SEQUENCE. I would like to use GENERATED ALWAYS AS IDENTITY.

ALTER TABLE public.patient ALTER COLUMN patientid Type int4 
USING patientid::int4 GENERATED ALWAYS AS IDENTITY;

Answer 1

Following the documentation

ALTER TABLE patient 
    ALTER patientid SET NOT NULL,  -- optional
    ALTER patientid ADD GENERATED ALWAYS AS IDENTITY 
        (START WITH 2);  -- optional

Add NOT NULL constraint if the column does not have the constraint yet. The optional clause START WITH start changes the recorded start value of the sequence.

Test it in DB<>Fiddle.