如何在oracle中创建触发器，以确保如果卫星日期早于当前日期，则不会删除任何记录？

Question

I have a table Movie.

create table Movie
(
mv_id number(5),
mv_name varchar2(30),
startdate date
)
insert into Movie values(1,'AVATAR','8-MAR-2012');
insert into Movie values(2,'MI3','20-MAR-2012');
insert into Movie values(3,'BAD BOYS','10-Feb-2012');

I want to create a trigger which will ensure that no records is deleted if satrt date is earlier than current date. My trigger code is --

create  or replace  trigger trg_1
before delete
on Movie
for each row
when(old.startdate < sysdate)
begin
raise_application_error(-20001, 'Records can not be deleted');
end;

The trigger is created.When I execute this code to delete data --

delete from Movie where mv_id=1;

Then the trigger fires but with errors, I don't know why I am getting such error. I dont want any error, i want only message. This is the error --

delete from Movie where mv_id=1
            *
ERROR at line 1: 
ORA-20010: Records can not be deleted
ORA-06512: at "OPS$0924769.TRG_1", line 3 
ORA-04088: error during execution of trigger 'OPS$0924769.TRG_1' 

I want to get rid of this error.

Answer 1

The only way for a trigger on a table to prevent any operation is throw an exception. You cannot have a trigger that prevents a DELETE and prints a message.

You can write a trigger that attempts to display a message and that allows the delete to happen

create  or replace  trigger trg_1
  before delete
  on Movie
  for each row
  when(old.startdate < sysdate)
begin
  dbms_output.put_line( 'Records can not be deleted');
end;

If the client happens to be configured to display the data written to the DBMS_OUTPUT buffer (most clients will not), this will display a message. But it will also allow the delete to be successful which does not sound like what you want.

Answer 2

Add exception handling in your delete statement.

All you need to do is write the delete statement in its own BEGIN and END block, and handle the exception there which is thrown from trigger. No row would be deleted if it matches the condition; however, the message to deny record deletion would be displayed using DBMS_OUTPUT command instead.

The below code does what you want:

BEGIN
DELETE FROM MOVIE WHERE MV_ID = 1;

EXCEPTION
  WHEN OTHERS
    THEN DBMS_OUTPUT.PUT_LINE('Records can not be deleted.');

END;

You can check the message in Output window. Below is the output I got in PL/SQL developer after executing above block of query.

Records can not be deleted.

I hope it helps.