SQL练习 2022/6/30

182. 查找重复的电子邮箱

题干
代码思路

1. group by 然后having判断数量大于一就可

select Email
from Person
group by email
having count(id)>1

2. 使用 GROUP BY 和临时表

select Email from
(
  select Email, count(Email) as num
  from Person
  group by Email
) as statistic
where num > 1;

183. 从不订购的客户

题干
代码思路

1. 嵌套查询

select name as Customers 
from Customers
where id not in
(
    select CustomerId 
    from Orders
)

2. 左连接，然后判断为null

select name as Customers 
from Customers left join Orders
on Customers.id= Orders.CustomerId
where CustomerID is null