Question

我正在尝试运行下面的脚本，但总是为名称字段获取NULL值。

SELECT u.name AS _user_name, s.name AS _school_name
FROM fwg_files AS f
LEFT JOIN users AS u ON u.id = f.user_id
LEFT JOIN user_profiles AS up ON up.user_id = u.id
LEFT JOIN school AS s ON s.id = up.profile_value

在JOIN ON学校表中我似乎遇到了这个问题，我尝试了SELECT s.id，它也返回NULL值。当我将最后一行更改为RIGHT JOIN时，它开始工作，但我只能看到s.name值，其他值为NULL

表fwg_files

id  | user_id
240 | 414
241 | 436

表用户

id  | name
414 | Name 1
436 | Name 2

表user_profiles

user_id | profile_value
414     | "6"
436     | "14"

表学校

id | name

 6 | School 1
14 | School 2

谢谢

Answer 1

如果必须使用profile_value列上的引文，则需要使用CONVERT和SUBSTRING_INDEX的混合来删除引号，以便能够执行操作。

检查以下代码：

SELECT users.name as User, s.name as School
  FROM fwg_files AS ff
  LEFT JOIN users ON users.id = ff.user_id
  LEFT JOIN user_profiles AS up ON up.user_id = users.id
  LEFT JOIN school AS s ON s.id = CONVERT(SUBSTRING_INDEX(up.profile_value,'"',-2),UNSIGNED INTEGER);

此处working SQL Fiddle code带引号。

SQL左/右JOIN问题

1 个答案: