Question

table user:
id_u*   f_name   l_name
----------------------
1       andi     mitchel
2       sarah    bench
3       kirsty   larx

table voucher:
id_v*   id_user    id_target
1       1          2
2       2          3

非常混淆如何将这些表与两个外键连接

$db->query("SELECT * FROM voucher v 
LEFT JOIN user u ON u.id_u = v.id_user
LEFT JOIN user u1 ON u1.id_u = v.id_target
WHERE .... ")

回显while循环...并且什么都不返回？

while($r = $q->fetch_array(MYSQLI_ASSOC)) :
   echo  $r['u.f_name'];
   echo  $r['u1.f_name'];
endwhile;

Answer 1

您的JOIN似乎绝对正确。唯一的问题是您已加入表user两次，因此您的列名称相同（例如f_name）。数据库将为这些列分配不同（但任意）的名称。您可以使用AS关键字覆盖此行为：

$db->query("SELECT v.*
                 , u.f_name AS user_f_name
                 , u.l_name AS user_l_name
                 , ta.f_name AS target_f_name
                 , ta.l_name AS target_l_name
            FROM voucher v 
            LEFT JOIN user u ON u.id_u = v.id_user
            LEFT JOIN user ta ON ta.id_u = v.id_target
            WHERE .... ")

然后：

while($r = $q->fetch_array(MYSQLI_ASSOC)) :
   echo  $r['user_f_name'];
   echo  $r['target_f_name'];
endwhile;

我认为你可以用（内部）LEFT JOIN替换JOIN s。除非您的id_user或id_target值引用了不存在的用户ID（id_u）。

Answer 2

看起来你要求所有在凭证表中的人，无论他们是在第1位（用户）还是位置2（目标）......然后，显示该人的姓名。

此查询对每个可能的人及其位置基础（通过WhichPosition）进行预查询。

SELECT STRAIGHT_JOIN
      AllVoucherUsers.WhatPosition,
      u.*
   FROM
      ( select distinct 
              v.id_user, 
             '1' as WhatPosition
           from voucher v
        union select distinct
             v.id_target as id_user,
             '2' as WhatPosition
           from voucher v
      ) AllVoucherUsers
      join users u
         on AllVoucherUsres.id_user = u.id_u

如果你只想要一个给定人的一个实例 - 无论他们的位置如何，只需删除所有“WhatPosition”引用的实例......

SELECT STRAIGHT_JOIN
      u.*
   FROM
      ( select distinct 
              v.id_user
           from voucher v
        union select distinct
             v.id_target as id_user
           from voucher v
      ) AllVoucherUsers
      join users u
         on AllVoucherUsres.id_user = u.id_u

Answer 3

SELECT * FROM voucher v 
    LEFT JOIN user u ON u.id_u = v.id_user OR u.id_u = v.id_target
    WHERE ....

Answer 4

怎么样： SELECT * FROM凭证JOIN用户ON id_user = id_u

更简单：

SELECT * FROM凭证，用户WHERE id_user = id_u

mysql加入两个表

4 个答案: