183. Customers Who Never Order没有下过订单的顾客MySQL

假设一个网站包含两个表，Customers表和Orders表。编写SQL查询以查找所有没订购任何商品的客户。

用到left outer join:

select c.name Customers from customers c
left outer join orders o on c.id=o.customerid
where customerid is null

或者直接找表中不存在的:

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

OUTER JJOIN总结：

1.Full outer join

a) A和B的并集。对于没有匹配的记录，会显示null。可以使用IFNULL判断。

SELECT * FROM TableA FULL OUTER JOIN TableB ON TableA.name = TableB.name

b) A表和B表没有交集的数据集

SELECT * FROM TableA FULL OUTER JOIN TableB ON TableA.name = TableB.name WHERE TableA.id IS null OR TableB.id IS null

2.Left outer join

a) A的完全集，而B表中匹配的则有值，没有匹配的则显示null

SELECT * FROM TableA LEFT OUTER JOIN TableB ON TableA.name = TableB.name

b)在A表中有而在B表中没有的集合

SELECT * FROM TableA LEFT OUTER JOIN TableB ON TableA.name = TableB.name WHERE TableB.id IS null

3. Right outer join

同left类似

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