update employees e1
set salary = (case when 'Seattle'=(select l.city
                                     from employees e, departments d, locations l
                                    where e.employee_id = e1.employee_id 
                                      and e.department_id = d.department_id
                                      and d.location_id = l.location_id) 
                   then salary+100
                   when 'Oxford'=(select l.city
                                    from employees e, departments d, locations l
                                   where e.employee_id = e1.employee_id 
                                     and e.department_id = d.department_id
                                     and d.location_id = l.location_id) 
                   then salary+200
               end)    
where employee_id IN (select employee_id
                        from employees e, departments d, locations l
                       where e.department_id = d.department_id
                         and d.location_id = l.location_id
                         and l.city IN ('Seattle','Oxford'));

UPDATE MY_TABLE
SET SALARY = CASE WHEN CITY = 'Sealte' THEN 100 WHEN CITY = 'Oxford' THEN 200 ELSE SALARY END
 WHERE CITY IN ('Sealte' , 'Oxford' )