1757. 可回收且低脂的产品
select product_id from Products
where low_fats="Y" and recyclable="Y"
584. 寻找用户推荐人
select name from customer
where referee_id !=2 or referee_id is NULL
直接用where函数即可,第二题也可使用isfull
select name from customer
where ifnull(referee_id,0)!=2
