MySQL: 为什么我用EXCEPT从原来的选择中排除一些行时出现错误？

Question

来源：Leetcode 1264

Table:友谊

+---------------+---------+
| Column Name   | Type    |
+---------------+---------+
| user1_id      | int     |
| user2_id      | int     |
+---------------+---------+
(user1_id, user2_id) is the primary key for this table.
Each row of this table indicates that there is a friendship relation between user1_id and user2_id.

Table：赞数

+-------------+---------+
| Column Name | Type    |
+-------------+---------+
| user_id     | int     |
| page_id     | int     |
+-------------+---------+
(user_id, page_id) is the primary key for this table.
Each row of this table indicates that user_id likes page_id.

编写一个 SQL 查询，使用您朋友喜欢的页面向 user_id = 1 的用户推荐页面。它不应该推荐您已经喜欢的页面。

Return 结果 table 任意顺序，无重复。

查询结果格式如下例：

Friendship table:
+----------+----------+
| user1_id | user2_id |
+----------+----------+
| 1        | 2        |
| 1        | 3        |
| 1        | 4        |
| 2        | 3        |
| 2        | 4        |
| 2        | 5        |
| 6        | 1        |
+----------+----------+
 
Likes table:
+---------+---------+
| user_id | page_id |
+---------+---------+
| 1       | 88      |
| 2       | 23      |
| 3       | 24      |
| 4       | 56      |
| 5       | 11      |
| 6       | 33      |
| 2       | 77      |
| 3       | 77      |
| 6       | 88      |
+---------+---------+

Result table:
+------------------+
| recommended_page |
+------------------+
| 23               |
| 24               |
| 56               |
| 33               |
| 77               |
+------------------+
User one is friend with users 2, 3, 4 and 6.
Suggested pages are 23 from user 2, 24 from user 3, 56 from user 3 and 33 from user 6.
Page 77 is suggested from both user 2 and user 3.
Page 88 is not suggested because user 1 already likes it.

我的代码：

select distinct(l1.page_id) as recommended_page
from Likes l1
left join Friendship f
on l.user_id = f.user2_id
where user1_id = 1


UNION

select distinct(l2.page_id) as recommended_page
from Likes l2
left join Friendship f
on l.user_id = f.user1_id
where user2_id = 1

EXCEPT 

select page_id as recommended_page
from Likes
where user_id = 1

虽然except附近有错误。有人可以帮我弄清楚为什么吗？非常感谢！

Answer 1

Leetcode 使用 MySQL 不支持 EXCEPT 关键字。

试试这个 SQL:

select recommended_page from
(
select distinct(l1.page_id) as recommended_page
from Likes l1
left join Friendship f
on l.user_id = f.user2_id
where user1_id = 1

UNION

select distinct(l2.page_id) as recommended_page
from Likes l2
left join Friendship f
on l.user_id = f.user1_id
where user2_id = 1
) x

WHERE recommended_page NOT IN
(
select page_id as recommended_page
from Likes
where user_id = 1
)

MySQL: 为什么我用EXCEPT从原来的选择中排除一些行时出现错误？

MySQL: Why there has an error when I use EXCEPT to exclude some rows from the original selection?

mysql

union

except