postgresql如何让主键自增

Sql代码  
CREATE TABLE customers  
(  
  customerid SERIAL primary key ,  
  companyname character varying,  
  contactname character varying,  
  phone character varying,  
  country character varying  
)  

Sql代码  
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');