如何查找使用另一個表作爲參考信息

Question

我有兩個表在mysql數據庫中，「點」和「用戶點」。 e.g

點：

id | pointvalue | message 
---------------------------- 
1 | 5   | comment 

Userpoints：

id | uid |pid | timestamp 
------------------------- 
89 | 5 | 1 | timestamp 

我將如何獲得在userpoints表點的總和時，它正受到userpoints的PID聯繫？

使用mysql查詢？

Answer 1

SELECT up.uid, SUM(p.pointvalue) total_points 
    FROM Userpoints up 
LEFT JOIN Points p 
     ON up.pid = p.id 
    WHERE up.uid = 5; 

Answer 2

select sum(p.pointvalue) from userpoints up left join points p on up.pid = p.id group by up.uid