1 题目描述
顾客表：Customers
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| customer_id | int |
| customer_name | varchar |
| email | varchar |
+---------------+---------+
customer_id 是这张表具有唯一值的列。 此表的每一行包含了某在线商店顾客的姓名和电子邮件。
联系方式表：Contacts
+---------------+---------+
| Column Name | Type |
+---------------+---------+
| user_id | id |
| contact_name | varchar |
| contact_email | varchar |
+---------------+---------+
(user_id, contact_email) 是这张表的主键（具有唯一值的列的组合）。 此表的每一行表示编号为 user_id 的顾客的某位联系人的姓名和电子邮件。 此表包含每位顾客的联系人信息，但顾客的联系人不一定存在于顾客表中。
发票表：Invoices
+--------------+---------+
| Column Name | Type |
+--------------+---------+
| invoice_id | int |
| price | int |
| user_id | int |
+--------------+---------+
invoice_id 是这张表具有唯一值的列。 此表的每一行分别表示编号为 user_id 的顾客拥有有一张编号为 invoice_id、价格为 price 的发票。
为每张发票 invoice_id 编写一个查询方案以查找以下内容：
customer_name：与发票相关的顾客名称。price：发票的价格。contacts_cnt：该顾客的联系人数量trusted_contacts_cnt：可信联系人的数量：既是该顾客的联系人又是商店顾客的联系人数量（即：可信联系人的电子邮件存在于Customers表中）。
返回结果按照 invoice_id 排序。
2 测试用例
输入：
Customers table:
+-------------+---------------+--------------------+
| customer_id | customer_name | email |
+-------------+---------------+--------------------+
| 1 | Alice | alice@leetcode.com |
| 2 | Bob | bob@leetcode.com |
| 13 | John | john@leetcode.com |
| 6 | Alex | alex@leetcode.com |
+-------------+---------------+--------------------+
Contacts table:
+-------------+--------------+--------------------+
| user_id | contact_name | contact_email |
+-------------+--------------+--------------------+
| 1 | Bob | bob@leetcode.com |
| 1 | John | john@leetcode.com |
| 1 | Jal | jal@leetcode.com |
| 2 | Omar | omar@leetcode.com |
| 2 | Meir | meir@leetcode.com |
| 6 | Alice | alice@leetcode.com |
+-------------+--------------+--------------------+
Invoices table:
+------------+-------+---------+
| invoice_id | price | user_id |
+------------+-------+---------+
| 77 | 100 | 1 |
| 88 | 200 | 1 |
| 99 | 300 | 2 |
| 66 | 400 | 2 |
| 55 | 500 | 13 |
| 44 | 60 | 6 |
+------------+-------+---------+
输出：
+------------+---------------+-------+--------------+----------------------+
| invoice_id | customer_name | price | contacts_cnt | trusted_contacts_cnt |
+------------+---------------+-------+--------------+----------------------+
| 44 | Alex | 60 | 1 | 1 |
| 55 | John | 500 | 0 | 0 |
| 66 | Bob | 400 | 2 | 0 |
| 77 | Alice | 100 | 3 | 2 |
| 88 | Alice | 200 | 3 | 2 |
| 99 | Bob | 300 | 2 | 0 |
+------------+---------------+-------+--------------+----------------------+
解释： Alice 有三位联系人，其中两位(Bob 和 John)是可信联系人。 Bob 有两位联系人, 他们中的任何一位都不是可信联系人。 Alex 只有一位联系人(Alice)，并是一位可信联系人。 John 没有任何联系人。
3 解题思路
- 关联
Contacts和Customers查询统计顾客的联系人数量contacts_cnt和 可信联系人的数量trusted_contacts_cnt
select co.user_id,
count(co.contact_email) as contacts_cnt,
count(co.contact_email) as trusted_contacts_cnt
from Contacts as co
inner join Customers as cu on co.contact_email = cu.email
group by co.user_id
执行结果
+-------+------------+--------------------+
|user_id|contacts_cnt|trusted_contacts_cnt|
+-------+------------+--------------------+
|1 |2 |2 |
|6 |1 |1 |
+-------+------------+--------------------+
- 关联
Invoices，Customers以及步骤 1 统计的顾客联系人数量信息，并对结果进行invoice_id排序，需要注意部分顾客没有联系人数量信息，是用ifnull()判断设置为 0
select i.invoice_id,
cu.customer_name,
i.price,
ifnull(contacts_cnt, 0) as contacts_cnt,
ifnull(trusted_contacts_cnt, 0) as trusted_contacts_cnt
from Invoices as i
inner join Customers as cu on i.user_id = cu.customer_id
left join (select co.user_id,
count(co.contact_email) as contacts_cnt,
count(co.contact_email) as trusted_contacts_cnt
from Contacts as co
inner join Customers as cu on co.contact_email = cu.email
group by co.user_id) as co on cu.customer_id = co.user_id
order by i.invoice_id;
执行结果
+----------+-------------+-----+------------+--------------------+
|invoice_id|customer_name|price|contacts_cnt|trusted_contacts_cnt|
+----------+-------------+-----+------------+--------------------+
|44 |Alex |60 |1 |1 |
|55 |John |500 |0 |0 |
|66 |Bob |400 |0 |0 |
|77 |Alice |100 |2 |2 |
|88 |Alice |200 |2 |2 |
|99 |Bob |300 |0 |0 |
+----------+-------------+-----+------------+--------------------+
