Postgres 13

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CREATE TABLE tbl (fips int, customer_id int);
INSERT INTO tbl VALUES 
  (17043, 2085)
, (17043, 2085)
, (42091, 4426)
, (42091, 4426)
;

CREATE TABLE

INSERT 0 4

TABLE tbl;

fips
customer_id

17043
2085

17043
2085

42091
4426

42091
4426

SELECT 4

ALTER TABLE tbl ADD COLUMN tbl_id serial PRIMARY KEY;

ALTER TABLE

TABLE tbl;

fips
customer_id
tbl_id

17043
2085
1

17043
2085
2

42091
4426
3

42091
4426
4

SELECT 4

-- start over, same demo for IDENTITY
ALTER TABLE tbl DROP COLUMN tbl_id;

ALTER TABLE

TABLE tbl;

fips
customer_id

17043
2085

17043
2085

42091
4426

42091
4426

SELECT 4

ALTER TABLE tbl ADD COLUMN tbl_id int GENERATED ALWAYS AS IDENTITY PRIMARY KEY;

ALTER TABLE

TABLE tbl;

fips
customer_id
tbl_id

17043
2085
1

17043
2085
2

42091
4426
3

42091
4426
4

SELECT 4