触发器小例子

触发器能进行一些约束.
这是个小例子,当Student表的StudentID列被发生更改时,BorrowStudent表的StudentID列也跟着更改.如果Student表删除某记录,BorrowStudent也删除对应StudentID的记录.

/*先删除将要创建而存在的表*/
drop table if exists Student;
drop table if exists BorrowStudent;

/*创建表*/
create table Student(
   StudentID int not null primary key,
   StudentName varchar(30) not null,
   StudentSex enum('m','f') default 'm'
)engine=myisam;

create table BorrowStudent(
   BorrowRecord int not null auto_increment primary key,
   StudentID int not null,
   BorrorDate date,
   ReturnDate date,
   foreign key(StudentID) references Student(StudentID)
)engine=myisam;

/*插入记录*/
insert into Student values(1235412,'java','m');
insert into Student values(3214562,'jiajia','m');
insert into Student values(5441253,'purana','f');

insert into BorrowStudent(StudentID,BorrorDate,ReturnDate)
                   values(1235412,'2007-01-01','2007-01-07');
insert into BorrowStudent(StudentID,BorrorDate,ReturnDate)
                   values(3214562,'2007-01-01','2007-01-07');
insert into BorrowStudent(StudentID,BorrorDate,ReturnDate)
                   values(5441253,'2007-01-01','2007-01-07');

/*创建触发器*/
delimiter $$
drop trigger if exists tduStudent$$
drop trigger if exists tddStudent$$
create trigger tduStudent before update
on Student for each row
begin
    if new.StudentID!=old.StudentID then
        update BorrowStudent
        set BorrowStudent.StudentID=new.StudentID
        where BorrowStudent.StudentID=old.StudentID;
    end if;
end$$

create trigger tddStudent before delete
on Student for each row
begin
    delete
    from BorrowStudent
    where BorrowStudent.StudentID=old.StudentID;
end$$
delimiter ;