oj-sql

题目

183. 从不订购的客户

考察点

1. 左连

solution

左连

左连接保证左表结果一定查出，关联右表数据null时，对应右列null

select c.name as `Customers`
from Customers c
left join Orders o
on (c.id=o.customerId)
where o.id is null;

子查询id not in

join查出的是有下单的顾客数据，not in进行反选

select name as `Customers`
from Customers
where id not in (
    select c.id
    from Customers c
    join Orders o
    on (c.id=o.customerId)
);

内部子查询可简化为单表查：

select name as `Customers`
from Customers
where id not in (
    select customerId
    from Orders
);