Adding primary key to existing table in Postgres

Primary_key_SQL

#!/usr/bin/env SQL

#Lets say you have a sec_sample table to which you want to add a id (surrogate) autoincremental PK:
ALTER TABLE sec_sample ADD column id serial;
UPDATE sec_sample SET id = nextval(pg_get_serial_sequence('sec_sample', 'id'));
ALTER TABLE sec_sample ADD PRIMARY KEY (id);