更新一列，如果空別的更新列b，否則如果兩個列的非空奈何

Question

這是我的表： 稱爲房間

+---------+---------+-----------+-------------+-------------+-------------+---------+ 
| 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 the id of the player who wants to join 

#room表：

1. if player_a_id！null and player_b_id！null then update nothing; if
2. player_a_id = null and player_b_id！null then update room set
3. player_a_id = $ player_id;如果player_b_id = null並且player_a_id！null，則更新房間設置player_b_id = $ player_id;

我目前的查詢（謝謝JW）（我試圖編輯它，但沒有用我失敗）。

 UPDATE room 
      SET  player_a_id = IF(player_a_id IS NULL OR player_a_id = 0 AND player_b_id != :chara_id, :chara_id, player_a_id), 
        player_b_id = IF(player_a_id != :chara_id AND player_b_id IS NOT NULL, :chara_id, player_b_id) 
      WHERE room_id = :room_id 

這是件好事，但如果兩者都爲空或= 0時更新兩列;我只想更新1

編輯：

這裏是樣品的結果： 後player_id 1聯接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 | 
+---------+---------+-----------+-------------+-------------+-------------+---------+ 

因爲這兩個字段爲空，然後它會同時更新的列我只想更新1列。

Answer 1

查看你的邏輯，你想更新player_a_id至$ player_id IF player_a_id = 0和player_b_id不= 0

而且要更新player_b_id至$ player_id IF player_b_id = 0和player_a_id是不是= 0

編輯 - 你也想更新player_a_id如果兩者都等於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 AND player_b_id=0, :player_id, player_b_id) 
WHERE room_id = :room_id