postgresql如何设置主键自增

CREATE TABLE customers 

( 

  customerid SERIAL primary key , 

  companyname character varying, 

  contactname character varying, 

  phone character varying, 

  country character varying

)

CREATE SEQUENCE event_id_seq 

START WITH 1

INCREMENT BY 1  NO MINVALUE  NO MAXVALUE 

CACHE 1;

alter table event alter column id set default nextval('event_id_seq');