如何更新MySQL中另一个表的聚合结果的字段？

Question

UPDATE a JOIN b ON a.app_id=b.app_id GROUP BY a.app_id SET 

remark_avg=AVG(b.score),remark_count=COUNT(b.id); 

以上基本上是我想要做的，但它不是一个有效的MySQL语句，如何正确写入它？

Answer 1

UPDATE a 
INNER JOIN (SELECT AVG(b.score) avg_score, 
        COUNT(b.id) cnt_id, 
        b.app_id 
       FROM b 
      GROUP BY b.app_id) x ON x.app_id = a.app_id 
     SET remark_avg = x.avg_score, 
      remark_count = x.cnt_id;