SQL练习 2022/7/5

584. 寻找用户推荐人

题干
代码思路

1. 嵌套查询

select name
from customer
where id not in(
    select id 
    from customer
    where referee_id =2
)

2. 直接查询

SELECT name FROM customer WHERE referee_id != 2 OR referee_id IS NULL;

586. 订单最多的客户

题干
代码思路

1. 分组排序，然后取第一条

select a.customer_number from
    (select  customer_number ,count(order_number) as cnt
    from Orders
    group by customer_number
    order by cnt desc
    limit 1)as a

2. 开窗函数

select customer_number from (
	select
		customer_number,
		dense_rank() over(order by count(*) desc) as rn
	from orders
    group by customer_number
) temp where rn = 1;

select a.customer_number from
    (select  customer_number,dense_rank() over(order by count(*) desc)as rn
    from Orders
    group by customer_number
    limit 1)as a