MySQL IF statement in UPDATE query

Question

Why cannot i use IF like this

UPDATE my_users
    IF(position2 = 18, "SET position = 17", ''),
    IF(position2 = 17, "SET position = 16", ''),
    IF(position2 = 16, "SET position = 15", ''),
        WHERE user_id => 170 AND user_id <= 1000

Only thing that works is

UPDATE my_users
    SET position2 = IF(position2 = 18, 17, 
                      IF(position2 = 17, 16, 
                        IF(position2 = 16, 15, ''
                      )
                        )
                          )
        WHERE user_id => 170 AND user_id <= 1000

is there way to make same thng in any other way because i have more than 40 conditions for that field position2 and its easy to get lost in this way?

Edit:

If i run

UPDATE my_users
    SET position2 = CASE position2  WHEN 18 THEN 17
                    WHEN 17 THEN 16
                    WHEN 16 THEN 15
                    END
            WHERE user_id => 170 AND user_id <= 1000

when position2 is anything not in CASE statement it sets it to nothing, how can i just keep any value other than what in CASE statement the same

Edit 2: I guess solution is:

   UPDATE my_users
        SET position2 = CASE position2  WHEN 18 THEN 17
                        WHEN 17 THEN 16
                        WHEN 16 THEN 15
                            ELSE position2
                        END
                WHERE user_id => 170 AND user_id <= 1000

Edit 3: If my request was not one time thing the more faster request would be as suggested Vatev

   UPDATE my_users
        SET position2 = CASE position2  WHEN 18 THEN 17
                        WHEN 17 THEN 16
                        WHEN 16 THEN 15
                        END
                WHERE user_id => 170 AND user_id <= 1000
                AND position2 >= 16 and <= 18

Answer 1

You can also do something like this:

                UPDATE my_users
                     SET position2 = CASE WHEN position2 =18 THEN 17
                        WHEN position2 =17 THEN 16
                        WHEN position2 =16 THEN 15 ELSE position2 
                        END
                WHERE user_id between 170 AND  1000
                AND position2 between 16 and  18