SELECT 
       * 
FROM 
       "tbl_Employees" 

WHERE
     ((UPPER("Last Name")    LIKE '%' || UPPER (:Var_LN) || '%'  ) OR (:Var_LN   IS NULL))
 and 
     ((      "Department-ID" =                  :Var_DPT        )  OR (:Var_DPT  IS NULL))
 and 
     ((      "Group-ID"      =                  :Var_GRP        )  OR (:Var_GRP  IS NULL))
 and 
     ((      "Start Date"    >                  :Var_StDtMn     )  OR (:Var_StDtMn IS NULL))
 and 
     ((      "Start Date"    <                  :Var_StDtMx     )  OR (:Var_StDtMx IS NULL))