mysql - SQL 插入与连接

Question

我的 mysql 查询有问题。

这是我的两张 table :

玩家位置:

ID |  playerid  | type | location
---|-----------------------

和用户:

ID  | playername | [..]
----|--------------------
 1  | example1   | ...

我想插入以下player_locations:

ID |  playerid  | type | location
---|-----------------------
 1 |     1      |  5   |  DOWNTOWN

这就是我的查询:

INSERT INTO player_locations (id, type, location)
  SELECT u1.ID as playerid,
        d.type,
        d2.location
  FROM users u1
  INNER JOIN users u2
    ON 1 = 1
  INNER JOIN (SELECT 5 as type
        FROM DUAL) d
  INNER JOIN (SELECT "DOWNTOWN" as location
        FROM DUAL) d2
    ON 1 = 1
  WHERE u1.playername = "example1";

但是当我在 users 中有 6 行时，它会在 player_locations 中插入 6 行相同的行

Answer 1

为什么不直接写这个呢？

INSERT INTO player_locations(id, type, location)
  SELECT u.ID as playerid, 5 as type, 'DOWNTOWN' as location
  FROM users u
  WHERE u.playername = 'example1';

自连接没有任何意义。您在查询中没有使用 u2 中的任何信息，因此它只是乘以行数。常量的额外连接是不必要的。