select product_idfrom Productswhere low_fats = 'Y' and recyclable = 'Y';

SQL 规定：null 的比较必须用 is null 或 is not null，不能用普通的等号（=）。

# Write your MySQL query statement below
select name
from Customer
where referee_id is null or referee_id != 2

# Write your MySQL query statement below
select name,population,area
from World
where area>=3000000 or population>=25000000

# Write your MySQL query statement below
select distinct author_id as id
from Views
where viewer_id = author_id
order by id

# Write your MySQL query statement below
select tweet_id as tweet_id
from Tweets
where LENGTH(content) > 15;