Trigger question

I have a dilema, and I'm not sure what the best way to go about solving it
is.  I have a table called employee_master, and a table called
employee_payroll.  The primary key for both tables is the employee name.  It
would be easy for me to set up a trigger that checked the master table when
inserting into the payroll table to make sure the employee exists. My
problem is this:  The employee names are stored Last, First in both tables.
The data file I am parsing to insert into the payroll table contains just
the last name.  Now I can make a trigger that uses a 'like' and checks the
employees division as well to ensure that they are in the employee_master,
but how do I update with just the last name?  I don't want to insert, I need
to be able to update the data in the row.

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