oracle中自增长的问题

SQL code

--创建学生表
create table student(
       stuid int  primary key , --主键，想设置为自增长
       sname varchar(10),
       sex   number(1),
       birthday date
       );

-- 建立序列：
　　create sequence stu_sequence
　　minvalue 1
　　maxvalue 99999999999999999999999999
　　start with 1
　　increment by 1
　　cache 20;

-- 建立触发器，写法1
  CREATE OR REPLACE TRIGGER "stu_trigger"  BEFORE
        INSERT ON  student  FOR EACH ROW 
        declare
        next_checkup_no number;
        begin
        select stu_sequence.nextval
        into next_checkup_no
        from dual;
        :NEW.stuid := next_checkup_no;
       end;
-- 建立触发器，写法2
create or replace trigger "stu_trigger" 
before insert on student for each row
       begin
         select stu_sequence.nextval into :new.stuid from dual;
       end;

--插入数据1
insert into student values(1,'riyun',0,sysdate);
--插入数据2
insert into student values('tom',0,sysdate);

问题：1、建立触发器的写法1和2有什么不同？
2、:NEW.stuid := next_checkup_no这里的“：”代表什么意思？
3、当执行插入数据1的时候oracle报“该触发器无效且未通过重新验证”错误？
4、当执行插入数据2的时候oracle报“没有足够的值”错误？


请大侠们帮忙看下指点迷津！！3q

------解决方案--------------------
1.写法不同而已，1中有局部变量，2中没有
2.:new中的:是一个整体，指新增的行，:=是指对变是赋值，也是一个整体
3.这个说明建立的触发器有问题
4.你的表有四列而你只入了三列,至少应是，这样列才匹配，但能不能执行是另一回事
insert into student(sname,sex,birthday) values('tom',0,sysdate);


------解决方案--------------------
CREATE OR REPLACE TRIGGER "TRI_DATAID" BEFORE INSERT ON "TIGER"."WEB_DATA" REFERENCING OLD AS "OLD" NEW AS "NEW" FOR EACH ROW
DECLARE last_Sequence NUMBER; last_InsertID NUMBER; factor NUMBER; BEGIN IF (:NEW."DATAID" IS NULL) THEN SELECT "DATA_0".NEXTVAL INTO :NEW."DATAID" FROM DUAL; ELSE SELECT Last_Number-1 INTO last_Sequence FROM User_Sequences WHERE UPPER(Sequence_Name) = UPPER('DATA_0'); SELECT :NEW."DATAID" INTO last_InsertID FROM DUAL; WHILE (last_InsertID > last_Sequence) LOOP SELECT "DATA_0".NEXTVAL INTO last_Sequence FROM DUAL; END LOOP; END IF; END;