PL_SQL模块学习之八、序列号与事务

1. 序列号

• 序列生成器在多用户环境下产生唯一的数字序列
• 独立于表的对象，由Oracle自动维护
• 同一个序列号可供多个用户（表）使用且相互独立

1.1 序列号创建与使用

EX:

SQL> create sequence stuseq  --创建一个序列号
  2  start with 2000
  3  increment by 1;
Sequence created

SQL> create table student(student_id number,name varchar2(20),sex char(2),birth_place varchar2(20));
Table created

SQL> desc student;
Name        Type         Nullable Default Comments 
----------- ------------ -------- ------- -------- 
STUDENT_ID  NUMBER       Y                         
NAME        VARCHAR2(20) Y                         
SEX         CHAR(2)      Y                         
BIRTH_PLACE VARCHAR2(20) Y                         

SQL> insert into student values(stuseq.nextval,'TOM','F','Boston');
1 row inserted

SQL> select * from student;
STUDENT_ID NAME                 SEX BIRTH_PLACE
---------- -------------------- --- --------------------
      2001 TOM                  F   Boston
-- ？？按理说应该是2000，为开始值
SQL> insert into student values(stuseq.nextval,'TOM2','F','Boston');
1 row inserted

SQL> select * from student;
STUDENT_ID NAME                 SEX BIRTH_PLACE
---------- -------------------- --- --------------------
      2001 TOM                  F   Boston
      2002 TOM2                 F   Boston

2. 事务

其原子性决定一个事务要么不做，要么做完

2.1 COMMIT

• 提交保证数据更改永久有效，其他用户也可看到一致数据
• 修改后未提交，重新登陆将失效，其他用户也看不到相关修改

2.2 ROLLBACK

修改后未提交，回滚可将数据恢复到原始数值
EX:

-- 查询数据
SQL> select sal from emp where empno = 7654;
      SAL
---------
  1250.00
-- 更新数据
SQL> update emp set sal=1000 where empno=7654;
1 row updated
-- 查询数据已更新
SQL> select sal from emp where empno = 7654;
      SAL
---------
  1000.00
-- 回滚
SQL> rollback;
Rollback complete
-- 数据已恢复
SQL> select sal from emp where empno = 7654;
      SAL
---------
  1250.00

2.3 SAVEPOINT

EX:

SQL> declare
  2   var_salary number(8,2);
  3  begin
  4   update emp set sal=1000 where empno=7654;
  5   select sal into var_salary from emp where empno=7654;
  6   dbms_output.put_line ('the first step salary is :'||var_salary);
  7  savepoint a1; #设置保留点a1
  8   update emp set sal=10001 where empno=7654;
  9   select sal into var_salary from emp where empno=7654;
 10   dbms_output.put_line ('the second step salary is :'||var_salary);
 11  savepoint a2;#设置保留点a2
 12   update emp set sal=10002 where empno=7654;
 13   select sal into var_salary from emp where empno=7654;
 14   dbms_output.put_line ('the third step salary is :'||var_salary);
 15  savepoint a3;#设置保留点a3
 16  rollback to a1;#回滚到保留点a1
 17  dbms_output.put_line ('the final step salary is :'||var_salary);
 18  end;
 19  /
the first step salary is :1000
the second step salary is :10001
the third step salary is :10002
the final step salary is :10002
PL/SQL procedure successfully completed

SQL> select sal from emp where empno = 7654;
      SAL
---------
  1000.00

rollback to a1后，语句块未结束，故事务也未结束，“PL/SQL procedure successfully completed”后查看数据，已修改完成