mysql - Mysql双连接更新

Question

我有三个表：对象，关系，点

关系表将多个点连接到对象。想象一下这样的表结构：

**OBJECTS**
  |object_id|object_name|object_points|
**RELATIONS**
  |relation_id|relation_object_id|relation_point_id|
**POINTS**
  |point_id|point_value(INT)|

不，我如何在 UPDATE 中进行“双重连接”以object_points从相关总和中更新对象表字段point_values？

Answer 1

像这样的东西？

UPDATE
        Objects AS o
    JOIN
        ( SELECT 
              r.relation_object_id
            , SUM(point_value) AS total_points
          FROM
                  Relations AS r
              JOIN
                  Points AS p
                ON p.point_id = relation_point_id 
          GROUP BY
              r.relation_object_id
        ) AS grp
      ON grp.relation_object_id = o.object_id
SET
    o.object_points = grp.total_points ;

Answer 2

这应该工作

Update OBJECTS set object_points=(select sum(POINTS.point_value) 
  from RELATIONS,POINTS where 
  RELATIONS.relation_object_id=OBJECTS.object_id 
  and RELATIONS.relation_point_id=POINTS.point_id group by relation_object_id)

这是工作演示：http ://sqlfiddle.com/#!2/d2a9f/1/0