2个sql表的组合

Question

我有以下表格（简化版）：

 CUSTOMERS
 -int id
 -varchar name

 ENVENTS
 -int id
 -int customer_id
 -int duration

我的问题是，我希望获得具有相同customer_id的持续时间总和，然后将customer_id替换为来自表客户的名称。

所以输出应该是：

+--------------+----------------+
| name         | sum(durtaions) |
+--------------+----------------+
| customername |          12313 |
+--------------+----------------+

• 这只是一个例子。

任何人都可以给我一个提示吗？

Answer 1

尝试连接两个表并使用GROUP BY子句来实现此目的：

SELECT c.name
      ,SUM(e.duration) AS duration
  FROM Customers c 
  JOIN EVENTS e 
    ON c.id = e.customer_id
 GROUP BY c.id

请参阅this sample SQLFiddle

---

编辑：

如果客户名称不在事件表

中，请尝试LEFT JOIN获取持续时间为0的客户名称

SELECT c.name
      ,IFNULL(SUM(e.duration),0) AS duration
  FROM Customers c 
  LEFT JOIN EVENTS e 
    ON c.id = e.customer_id
 GROUP BY c.id

请参阅this SQLFiddle

查看JOINs in MySQL和A Visual Explanation of SQL Joins

Answer 2

请尝试以下代码。

select c.name,SUM(e.duaration) as duration 
from customer as c 
left join events as e 
    on c.id = e.id 
group by e.customer_id