Oracle triggers

1. Trigger Overview

• The essence of trigger is a stored procedure , seeing the name of a thing one thinks of its function , When a specific event occurs Oracle Will execute the code in the trigger ( and java The monitor inside is a little similar );
• Specific events ： Is to execute the update DML and DDL sentence ;
• Triggers cannot be explicit ( Manual ) Called ;
• Components of trigger ：

1. Trigger statements （ event ）： Define the... That activates the trigger DML Events and DDL event ;
2. Trigger limit （when）： Conditions for executing triggers , This condition must be true to activate the trigger ;
3. Trigger action （ The main body ）： It is the specific event to express after the trigger is triggered , stay begin and end Between sql.

Classification of triggers ：

1. Divide... In time ：After trigger and Before trigger Two kinds of ;
2. From the modification of data ：insert、delete、update Three ;
3. Divide from the scope of application ： Row level triggers and table level triggers .

2. How triggers work

• After trigger ：
  Post trigger , That is, after the data of the table is updated , Note that it is not after saving .

• Before trigger ：
  Pre trigger , That is, trigger before the data of the table is updated .

• Trigger before adding operation , The modification and deletion operations are triggered after .

3. A temporary table -:new and :old

• When using triggers Oracle The temporary table will be automatically created according to the type of trigger used -:new perhaps :old, The fields in the table are consistent with those in the currently operated table .

• A temporary table “:new”： When adding Oracle The temporary table will be automatically created , It is used to temporarily store the added data . After confirming the added data , Then insert the data in the temporary table into the real table . Wait until the add operation is submitted , The temporary table disappears automatically .

• A temporary table “:old”： Delete 、 When modifying operations Oracle The temporary table will be automatically created , It is used for temporary storage and deletion 、 Data before modification . Delete, etc 、 After the modification operation is submitted , The temporary table disappears automatically .

4. Create trigger Syntax

create or replace trigger  Trigger Name            -- Trigger Name 
after insert            -- Post trigger , And it is to add type trigger 
on  Table name                  -- The table on which the trigger depends 
for each row            -- Line level triggers 
begin                                                         
        if :new. Field  = 0 then                  -- Trigger conditions 
        dbms_output.put_line(' Warning ： Record inserted , But the quantity is zero ');
		raise_application_error(-20018,' Warning ： Record inserted , But the quantity is zero ！'); -- Throw an exception ,-20018 It's an error code , The scope is -20000 To -21000.
        end if;
end;

5 trigger

5.1 Data script

-- Create a student information table 
create table stuInfo
(
  stuNo varchar2(8) not null primary key,
  stuName varchar2(10) not null,
  stuSex varchar2(2) not null,
  stuAge number(6) not null,
  stuSeat number(6) not null,
  strAddress varchar2(255) default(' The address is unknown ')
)


-- Create a student transcript 
create table stuMarks
(
  ExamNo varchar2(7) not null primary key,
  stuNo varchar2(6) not null references stuInfo(stuNo),
  writtenExam number(6) null,
  LabExam number(6) null
)

-- Insert test data into the student information table 
insert into stuInfo(stuNo, stuName, stuSex, stuAge,stuSeat,strAddress)
select 's25301', ' Zhang Qiuli ', ' male ', 18,1, ' Beijing haidian ' from dual union
select 's25303', ' Lisbon ', ' Woman ', 22,2, ' Heyang Luoyang ' from dual union
select 's25302', ' Li Wencai ', ' male ', 85, 3,' The address is unknown ' from dual union
select 's25304', ' Ouyang Junxiong ', ' male ', 28, 4,' xinjiang ' from dual union
select 's25318', ' Mei Chaofeng ', ' Woman ', 23, 5,' The address is unknown ' from dual


insert into stuInfo(stuNo, stuName, stuSex, stuAge,stuSeat,strAddress)
select 's25322', ' Publicize a', ' male ', 18,1, ' Beijing haidian ' from dual


-- Insert test data into the student transcript 
insert into stuMarks(ExamNo, stuNo, writtenExam, LabExam)
select 's271811', 's25303', 93, 59 from dual union
select 's271813', 's25302', 63, 91 from dual union
select 's271816', 's25301', 90, 83 from dual union
select 's271817', 's25318', 63, 53 from dual



select * from stuinfo;
select * from stumarks;

5.2 Post trigger -delete operation

-- Lisiwen cannot be deleted .
create or replace trigger tname       -- Trigger Name 
after delete                          -- Then trigger ( Delete class )
on stuinfo                            -- Table name 
for each row                          -- Line level triggers 
begin
       -- From the old watch (:old) Get the name of the deleted student and judge 
       if :old.stuname=' Lisbon ' then
       raise_application_error(-20018,' This student cannot be deleted ！'); -- Throw an exception 
    end if;
end;

5.3 Pre trigger insert& Post trigger delete& Post trigger &update

-- Liswen cannot be deleted , Zhang San cannot add , Mei Chaofeng cannot modify .
create or replace trigger tname
after delete or insert or update
on stuinfo
for each row
begin
    if deleting then-- Delete 
    -- From the old watch (:old) Get the name of the deleted student and judge 
    if :old.stuname=' Lisbon 'then
      raise_application_error(-20018,' This student cannot be deleted ！');
    end if;
    elsif inserting then-- increase 
    -- From the new table (:new) Get the names of the added students and make judgments 
    if :new.stuname=' Zhang San ' then
      raise_application_error(-20019,' The student cannot ！');
    end if;
    elsif updating then-- modify 
    -- From the old watch (:old) Get the name of the student to be modified and judge 
    if :old.stuname=' Mei Chaofeng ' then
      raise_application_error(-20020,' This student cannot modify ！');
    end if;
    end if;
end;

5.4 Multi table deletion

-- Delete students and automatically delete their grades .
create or replace trigger tname
after delete
on stuinfo
for each row
declare     -- Define variables to save the student ID to be deleted ( It is also possible not to define )
sno varchar2(10);
begin
    -- Get the student ID of the deleted student 
    sno:=:old.stuno;
    delete from stumarks where stuno=:old.stuno; --( Or write stuno=sno)
end;

5.4 trigger + Sequence = Identity column

create sequence sname_01;  -- Create sequence 

-- Create trigger 
create or replace trigger tname
before insert
on  Table name 
for each row
begin
    :new.sid:=sname_01.nextval;
end;

6. Case study

select * from stuinfo;
select * from stumarks;

--1、 In the student information form stuinfo Set trigger on ： Liswen cannot be deleted ;
create or replace trigger  a_1
after delete
on stuinfo 
for each row 
begin
  if:old.stuname=' Lisbon ' then
  raise_application_error(-20018,' Liswen cannot be deleted ');
  end if;
end;

delete from stuinfo where stuname=' Lisbon ';

--2、 In the student information form stuinfo Set trigger on ： Fools cannot increase ;
create or replace trigger a_2
after insert
on stuinfo
for each row 
begin
  if:new.stuname=' Idiot 'then
   raise_application_error(-20019,' Fools cannot increase ');
   end if;
end;

insert into stuinfo values('s25324',' Idiot ',' male ',18,20,' Hunan changsha ');
--3、 In the student information form stuinfo Set trigger on ： Li Wencai cannot modify ;
create or replace trigger a_3
after update
on stuinfo 
for each row 
begin
  if:old.stuname=' Li Wencai 'then
  raise_application_error(-20020,' Li Wencai cannot be modified ');
  end if;
end;

update stuinfo set stuname=' Li doubi ' where stuname=' Li Wencai ';


--4、 Create a new item table ( Field customization ), Set the trigger to realize the automatic growth of commodity number .
create table zdy(
  sid number,
  sname varchar2(10)
)

-- Create sequence 
create sequence sid_1
-- Create trigger 
create trigger t_4
before insert
on zdy
for each row
begin
    :new.sid:=sid_1.nextval;
end;
-- increase 
insert into zdy(sname)
values(' Li Si ')

select * from zdy

7. summary

• Triggers are stored procedures that are executed automatically when a specific event occurs
• Triggers are divided into dml trigger 、ddl There are three types of triggers and database level triggers
• dml The three types of triggers include row level triggers 、 Statement level triggers and instead of trigger
• Some commonly used built-in packages ：

1. dbms_output Package output pl/sql Debugging information of the program
2. dbms_lob Packages provide operations lob Subroutine of data
3. dbms_xmlquery Convert query results to xml Format
4. dbms_random Provide random number generator
5. utl_file Used to read and write operating system text files