sql
create table Customers (
    id int primary key ,
    name varchar(20)
);

create table Orders (
    id int primary key ,
    customeriId int ,
    foreign key (customeriId) references Customers(id)
);

insert into Customers values (1,'Joe'),(2,'Henry'),(3,'Sam'),(4,'Max');
insert into Orders values (1,3),(2,1);

select name as Customers
from Customers as c 
where c.id not in 
(select customeriId from Orders);

select name as Customers 
from Customers left join Orders 
on Customers.id = Orders.customeriId  
where Orders.customeriId is null;

总结

• 左连接会返回左表所有记录和右表匹配记录