php - Update a column a if null else update column b else if both column's are not null do nothing -

this table: called room

+---------+---------+-----------+-------------+-------------+-------------+---------+ | room_id | room_no | room_stat | room_name   | player_a_id | player_b_id | turn_of | +---------+---------+-----------+-------------+-------------+-------------+---------+ |       1 |       1 |         0 | blah        |           0 |           0 |       0 | |       2 |       5 |         0 | second room |           0 |           0 |       0 | |       3 |       3 |         0 | 3rd room    |           0 |           0 |       0 | |       4 |       4 |         0 | 4th room    |           0 |           0 |       0 | +---------+---------+-----------+-------------+-------------+-------------+---------+  $player_id //contains id of player wants join 

#room table:

1. if player_a_id !null , player_b_id !null update nothing; if
2. player_a_id = null , player_b_id !null update room set
3. player_a_id = $player_id; if player_b_id = null , player_a_id !null update room set player_b_id = $player_id;

my current query(thanks jw)(i tried edit no avail failed).

     update  room              set     player_a_id = if(player_a_id null or player_a_id  = 0 , player_b_id != :chara_id, :chara_id, player_a_id),                      player_b_id = if(player_a_id != :chara_id , player_b_id not null, :chara_id, player_b_id)                room_id  = :room_id 

this updates 2 columns if both empty or = 0; want update 1.

edit:

here sample result: after player_id 1 joins room_id 4:

+---------+---------+-----------+-------------+-------------+-------------+---------+ | room_id | room_no | room_stat | room_name   | player_a_id | player_b_id | turn_of | +---------+---------+-----------+-------------+-------------+-------------+---------+ |       1 |       1 |         0 | blah        |           0 |           0 |       0 | |       2 |       5 |         0 | second room |           0 |           0 |       0 | |       3 |       3 |         0 | 3rd room    |           0 |           0 |       0 | |       4 |       4 |         0 | 4th room    |           1 |           1 |       0 | +---------+---------+-----------+-------------+-------------+-------------+---------+ 

since both columns null updates both of column want update 1 column.

reviewing logic, want update player_a_id $player_id if player_a_id = 0 , player_b_id not = 0.

and want update player_b_id $player_id if player_b_id = 0 , player_a_id not = 0.

edit -- want update player_a_id if both equal 0.

update  room set     player_a_id = if(player_a_id=0, :player_id, player_a_id),     player_b_id = if(player_a_id!=0 , player_b_id=0, :player_id, player_b_id)   room_id  = :room_id