mysql - Mysql update if（一些语句）

Question

我只想在表不包含指定值时创建一个 mysql 更新（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;

sqlfiddle 中的代码

我希望这只能是一个mysql问题

Answer 1

一种方法是使用反连接模式，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 寻找匹配的行，但如果没有找到匹配的行，那么 MySQL 将生成一个包含所有 NULL 值的行。我们可以使用谓词过滤掉任何匹配的行q.uid IS NULL，所以我们只会得到不匹配的行。

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