如何使用另一个表作为参考来查找信息

Question

我在mysql数据库中有两个表，"points“和"userpoints”。e.g

点数：

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

用户点：

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

当userpoints中的pid链接到userpoints表时，我如何获得userpoints表中的点数总和？

使用mysql查询？

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