mysql的更新，如果（某些語句）

Question

我想創建一個MySQL更新僅在表不包含指定的值（如果玩家有空槽）

我的想法：

update player_items 
set slot=20 
where id=0 
and uid=2 
and (SELECT COUNT(*) 
    FROM player_items 
    WHERE slot=12 and uid=2) = 0; 

code in sqlfiddle

我希望這可以只有一個mysql問題

Answer 1

一種方法是使用反連接模式，例如

UPDATE player_items p 
    LEFT 
    JOIN (SELECT uid 
      FROM player_items 
      WHERE slot = 12 
      AND uid = 2 
      LIMIT 1 
     ) q 
    ON q.uid = p.uid 
    SET p.slot = 20 
WHERE p.id = 0 
    AND p.uid = 2 
    AND q.uid IS NULL 

LEFT JOIN查找匹配的行，但如果找不到匹配的行，那麼MySQL將生成一個包含所有NULL值的行。我們可以使用q.uid IS NULL謂詞過濾掉任何匹配的行，因此我們只會在沒有匹配的情況下獲取行。

mysql> UPDATE player_items p 
    -> LEFT 
    -> JOIN (SELECT uid 
    ->   FROM player_items 
    ->   WHERE slot = 12 
    ->    AND uid = 2 
    ->   LIMIT 1 
    ->  ) q 
    ->  ON q.uid = p.uid 
    -> SET p.slot = 20 
    -> WHERE p.id = 0 
    -> AND p.uid = 2 
    -> AND q.uid IS NULL 
    -> ; 
Query OK, 0 rows affected (0.00 sec) 
Rows matched: 0 Changed: 0 Warnings: 0