you have the idea of the link backwards
list-brokerage-by-category.php?Department=<?php echo $row_rsShowCats['Job_Department']; ?>
this creates a querystring variable named Department that is given the value of the department name from the recordset.
for the recordset, set the where clause to:
WHERE departments = paramDepartment
then create a new variables as:
Default Value: -1
Run Time Value: $_GET['Department']