postgresql如何让主键自增

1. 
   法一：
   
   Sql代码  收藏代码
   CREATE TABLE customers
   (
     customerid SERIAL primary key ,
     companyname character varying,
     contactname character varying,
     phone character varying,
     country character varying
   )
    法二：
   
   Sql代码  收藏代码
   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');