Simple trigger question

Howdy,

sorry, but I have no manuals solving the following problems. I have two tables.
1) Table of departments with department-name,location and number 2) Table of persons with name,address and department number

Now, I would like to delete a department and set without using referential integrity all persons department number which corresponds to the deleted department to NULL.

I would like to solve it with the creation of a trigger. Can someone send me the call of the correct "create trigger"-statement with full pl-sql-block (also no manual available).

Thanx in advance and sorry about this stupid question, Timo.

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