输出产品清单以及谁购买了这些产品

Question

与建议的表格一样，我希望输出所有产品的清单，但同时列出谁购买了每个产品。

我的数据结构如下

products
=================
|prodId |prod   |
=================
|1      |hat    |
|2      |shirt  |
|3      |watch  |
|4      |cream  |
|5      |pizza  |
=================


orders
=========================
|ordID  |prodID |cust   |
=========================
|1      |2      |paul   |
|2      |4      |paul   |
|3      |4      |bob    |
|4      |5      |jane   |
|5      |4      |sarah  |
=========================

我希望收到这个结果，如果没有客户购买，那将是null

=================
|prod   |cust   |
=================
|hat    |null   |
|shirt  |paul   |
|watch  |null   |
|cream  |paul   |
|cream  |bob    |
|cream  |sarah  |
|pizza  |jane   |
=================

Answer 1

SELECT p.prod, o.cust, count(o.*) count_of
FROM products as p
LEFT OUTER JOIN orders as o ON p.prodid = o.prodid
GROUP BY prod, cust

要获得以from products开头的al产品列表，请使用OUTER联接，该联接允许返回不匹配的行以及匹配的行。

然后group by将列表缩减为唯一组合，并启用聚合这样的计数。

Answer 2

以下查询应该完成工作

select p.prod, o.cust from products p left outer join orders o on p.prodId = o.prodId;