PL/SQL 设置主键自增长

假设关键字段为id，建一个序列，代码为： 
create sequence seq_user_custom 
minvalue 1 
maxvalue 99999999 
start with 1 
increment by 1 
nocache 
order; 
创建触发器代码为： 
CREATE OR REPLACE TRIGGER trg_seq_user_custom 
BEFORE INSERT ON user_custom 
FOR EACH ROW 
BEGIN
SELECT   seq_user_custom.nextval INTO :new.id FROM dual; 
END;