Reverse of Cascade Delete

I have a emp (Child) table and Dept (master) table. Cascade delete is set between two. I want to have cascade delete on e.g. If some body delete a Dept records all related records in Emp should be deleted. Its is working fine for me. Now what I want in addition to this is whenever user what to child record in Emp table it should not be deleted if there is a master record in DEPT.

Can some body suggest the ways.

Regards,

MAB
qaziasifAsked:
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DrJekyllCommented:
You could write a Pre-Delete trigger on your child table that checks to see if
a parent record exist. If it does then rollback the delete.
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qaziasifAuthor Commented:
Can u give me some example
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qaziasifAuthor Commented:
I wan to do this in Database not on FORMS
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DrJekyllCommented:
Correction, you cannot use a trigger as transaction control statements are not allowed.  I am not sure how to do this at the database level which is ideal.  In your app you could check to see if a parent record exist. If so do not allow deletion. However this not prevent deletions outside of the application.  
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anand_2000vCommented:
Create or replace trigger trg_emp before delete on emp as
declare
a number;
begin
select count(*) into a from dept where deptno=:old.deptno;
if a <>0 then
raise_application_error(-20111,'Delete not allowed');
end if;
end;
/
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qaziasifAuthor Commented:
Think carefully.

If I have set Cascade Detele on Dept Table (Which I mentioned above) and want to delete data from dept table where deptno = 10 then the trigger you mentioned would fire and my Cascade would not work. If I adopt the way by Anand then I need to know how to by pass the trigger code when its is being called by some other source
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andrewstCommented:
Strange requirement.  But if you have a foreign key from EMP to DEPT, then if EMP.DEPTNO is not null it MUST mean that the associated DEPT record exists.  So you can prevent delete like this:

CREATE OR REPLACE TRIGGER strange_rule BEFORE DELETE ON emp
FOR EACH ROW
BEGIN
  IF :old.deptno IS NOT NULL THEN
    RAISE_APPLICATION_ERROR(-20001,'Cannot delete employee that is related to a department' );
  END IF;
END;
/

Then you still have the problem of the cascade delete on DEPT being stopped by this.  The only way round that (I can think of) is to set a flag variable in a package like this:

CREATE OR REPLACE PACKAGE pkg IS
  deleting_dept BOOLEAN := FALSE;
END;
/

CREATE OR REPLACE TRIGGER dept_bd
BEFORE DELETE ON dept
BEGIN
  pkg.deleting_dept := TRUE;
END;
/

CREATE OR REPLACE TRIGGER dept_ad
AFTER DELETE ON dept
BEGIN
  pkg.deleting_dept := FALSE;
END;
/

Then change the first trigger to:

CREATE OR REPLACE TRIGGER strange_rule BEFORE DELETE ON emp
FOR EACH ROW
BEGIN
  IF :old.deptno IS NOT NULL AND NOT pkg.deletin_dept THEN
    RAISE_APPLICATION_ERROR(-20001,'Cannot delete employee that is related to a department' );
  END IF;
END;
/

How about that?
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qaziasifAuthor Commented:
It seems okay to me. Its a nice way of doing reverse cascade.

 About strange, if you have to maintain old stuff and you cant do much about it then you have to do like this. Anyway thank you for propmt response.

Regards,

MAB
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