Q:

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

Credit: Leetcode 1264
Table: Friendship
+---------------+---------+
| 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: Likes
+-------------+---------+
| 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.
 

Write an SQL query to recommend pages to the user with user_id = 1 using the pages that your friends liked. It should not recommend pages you already liked.
Return result table in any order without duplicates.
The query result format is in the following example:
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.

My code:
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

While there has an error near except. Can someone help me to figure out why? Thank you so much!

A:

Leetcode uses MySQL which does not support the EXCEPT keyword.
Try this 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
)

