oracle通过序列来实现主键的自增

建表语句：

CREATE TABLE tb_user(
ID VARCHAR2(20) PRIMARY KEY,
NAME VARCHAR2(20)
);

创建序列：
CREATE SEQUENCE seq_tb_user
START WITH 1
INCREMENT BY 1;

创建触发器：
CREATE OR REPLACE TRIGGER TG_TB_USER
BEFORE INSERT ON TB_USER
FOR EACH ROW
BEGIN
SELECT SEQ_TB_USER.NEXTVAL INTO :NEW.ID FROM DUAL;
END;

测试：
INSERT INTO TB_USER (NAME) VALUES ('yyb');

SELECT * FROM TB_USER;