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

Answer 1

Suppose you have a table patient previously created as

CREATE TABLE patient( patientid int, col1 int );

and a row inserted as

INSERT INTO patient VALUES(1,5);

Firstly create a sequence starting +1 iterated from the max value of ID and make it default for your column

CREATE SEQUENCE mySeq START WITH 2;
ALTER TABLE patient ALTER COLUMN patientid SET DEFAULT nextval('mySeq');

and convert your column to a primary key

ALTER TABLE patient ALTER COLUMN patientid SET NOT NULL;
ALTER TABLE patient ADD CONSTRAINT uk_patientid UNIQUE (patientid);

whenever you insert new rows such as

INSERT INTO patient(col1) VALUES(10);
INSERT INTO patient(col1) VALUES(15);

you'll observe that you sucessfully made your column as an identity column

SELECT * FROM patient

patientid  col1
---------  ----
1          5
2          10
3          15