Experiment 7 trigger

One . The experiment purpose

（1） Understand the purpose of triggers 、 Type and working principle .
（2） Master the use of T-SQL Statement to create and maintain triggers .
（3） Master the use of SQL Server Management Studio establish 、 Methods of maintaining triggers .

Two . Experimental content

1. establish AFTER trigger

   （1） Create a trigger that triggers when inserting sc_insert, Direction SC Table insert data , Ensure that the student number inserted is already in Student Presence in table , And also make sure that the inserted course number is in Course Presence in table ; If it does not exist , Then corresponding prompt information will be given , And cancel the insert operation , The prompt information is required to indicate whether the student number does not meet the conditions or the course number does not meet the conditions .（ notes ：Student Table and SC The foreign key constraint of the table must be cancelled first .）

   if exists(select name from sysobjects where name='sc_insert' and type='TR')
   	drop trigger sc_insert
   go 
   
   CREATE TRIGGER sc_insert
      ON SC
      AFTER INSERT
   AS 
   BEGIN
   	declare @sno char(8),@cno char(8)
   	select @sno=Sno , @cno=Cno
   	from inserted
   	if not exists(select Sno from Student where @sno=Sno)
   		begin
   			rollback tran
   			print(' The student number doesn't exist ')
   		end
   	else if not exists(select Cno from Course where @cno=Cno)
   		begin
   			rollback tran
   			print(' Course number does not exist ')
   		end
   END
   GO
   

   

   （2） by Course Table to create a trigger Course_del, When deleted Course A course information in the table when , At the same time, the table SC The corresponding student course selection records in the are also deleted .

   CREATE TRIGGER Course_del
      ON  Course
      AFTER DELETE
   AS 
   begin transaction
   	declare @cno char(8)
   	select @cno=Cno
   	from deleted
   	delete from SC
   	where [email protected]
   commit transaction
   GO
   

   （3） stay Course Add an GPa to the table avg_grade Field （ Record the average grade of each course ）, Create a trigger grade_modify, When SC When the grade of a student in the table changes , be Course The average score in the table can also be changed in time .

   alter table Course add avg_grade int;
   
   update Course 
   set avg_grade=(
   	select avg(Grade) from SC where SC.Cno=Course.Cno);
   

   

   CREATE TRIGGER grade_modify
      ON  SC
      AFTER UPDATE
   AS 
   BEGIN
   	if update(Grade)
   		begin
   			update Course
   			set avg_grade=(
   				select avg(Grade) from SC
   				where SC.Cno=Course.Cno
   				group by Cno)
   		end
   END
   GO
   

   （4） Test the above three triggers .
   1. test sc_insert trigger

   insert into SC values('20220002','011',99)
   

   
   Delete foreign key constraint

   alter table SC
   drop constraint FK__SC__Sno__6477ECF3;
   alter table SC
   drop constraint FK__SC__Cno__656C112C;
   

   

   insert into SC values('20110002','006',71);
   

   

   insert into SC values('20110005','001',71);
   

   

   insert into SC values('20110002','011',71);
   

   
   2. test Course_del trigger

   delete from Course
   where Cno='010';
   

   
   3. test grade_modify trigger

   update SC
   set Grade=70
   where Sno='20110001' and Cno='001';
   

   

2. establish INSTEAD OF trigger
   （1） Create a view student_view, Including student ID 、 full name 、 Course no. 、 Course name 、 Performance and other attributes , stay student_view Create a trigger on grade_modify, When the student_view When modifying the grades of students in , What is actually modified is SC The corresponding record in .
   Create view

   create view student_view
   as
   	select Student.Sno,Sname,Course.Cno,Cname,Grade
   	from Student,Course,SC
   	where Student.Sno=SC.Sno
   	and Course.Cno=SC.Cno;
   

   Create trigger

   CREATE TRIGGER grade_modify2
      ON student_view
      INSTEAD OF UPDATE
   AS 
   BEGIN
   	if update(Grade)
   		begin
   			update student_view
   			set Grade=(select Grade from inserted)
   			where Sno=(select Sno from inserted)
   			and Cno=(select Cno from inserted)
   		end
   END
   GO
   

   （2） stay Student Insert a getcredit Field （ Record the credits obtained by students ）, Create a trigger ins_credit, When changed （ notes ： Including insertion ）SC The student's grades in the table , If the new score is greater than or equal to 60 branch , Then the student can get credits for this course , If the new score is less than 60 branch , The student fails to get credit for this course .

   -- Add fields 
   alter table Student add getcredit tinyint;
   go
   
   -- to update Student surface 
   update Student 
   set getcredit=(
   	select sum(Credit) from Course,SC
   	where Course.Cno=SC.Cno and Student.Sno=SC.Sno)
   go
   
   -- Create trigger 
   CREATE TRIGGER ins_credit
      ON  SC
      AFTER UPDATE,INSERT
   AS 
   BEGIN
   	declare @sno char(8),@credit tinyint
   	select @sno=Sno,@credit=Credit
   	from Course,inserted
   	where Course.Cno=inserted.Cno and inserted.Grade>=60
   	update Student 
   	set [email protected]
   	where [email protected]
   END
   GO
   

   （3） Test the above two triggers .
   1. test grade_modify2

   update student_view
   set Grade=50
   where Sno='20110002' and Cno='001'
   

   
   2. test ins_credit trigger

   update SC
   set Grade=59
   where Sno='20110001' and Cno='002';
   

   

   Here is the addition or insertion of data after the table structure is modified Student Effective in the table

3. Use T-SQL Statement management and maintenance triggers
   （1） Use system stored procedures sp_helptrigger Check triggers grade_modify Information about .

   sp_helptrigger 'student_view';
   

   

   （2） Use sp_helptext Check triggers grade_modify Definition in .

   sp_helptext 'grade_modify2';
   

   

   （3） Use select Statement view trigger grade_modify The definition of .

   select name,text from sysobjects,syscomments 
   where sysobjects.id=syscomments.id 
   	and name='grade_modify2' and type='TR'
   

   

   （4） Use system stored procedures sp_depends Check triggers grade_modify The relevance of （ That is, which basic tables are involved in the trigger ）.

   sp_depends 'grade_modify2';
   

   

   （5） take sc_insert The trigger is changed to instead of trigger , The functions realized remain unchanged .

   drop trigger sc_insert;
   
   
   CREATE TRIGGER sc_insert
      ON  SC
      INSTEAD OF INSERT
   AS 
   BEGIN
   	declare @sno char(8),@cno char(8)
   	select @sno=Sno , @cno=Cno
   	from inserted
   	if not exists(select Sno from Student where @sno=Sno)
   		begin
   			rollback tran
   			print(' The student number doesn't exist ')
   		end
   	if not exists(select Cno from Course where @cno=Cno)
   		begin
   			rollback tran
   			print(' Course number does not exist ')
   		end
   END
   GO
   

   test :
   

   （6） Set the trigger sc_insert Delete .

   drop trigger sc_insert;
   

   

4. Use SQL Server Management Studio Manage triggers
   （1） stay SQL Server Management Studio Recreate the trigger you just deleted sc_insert.
   Expand the database where you want to create the trigger , In the database directory, select the table to create the trigger , Expand data table , Right click... Under the data table node " trigger ", And then choose " New trigger "
   
   Write the statement of the trigger in this position :
   
   as follows :
   
   Finally, execute to create a successful

   （2） Check triggers sc_insert The content of .
   
   

   （3） Delete trigger sc_insert.