求助~数据库中两表键值的传递

我建了两张表class,studnet.
classname|score                 studentname|class|score
a                                                 1                        a      60
b                                                 2                        b       70
c                                                 3                        c       80
                                                   4                        a      70
如何让CLASS中的SCORE列的值是STUDENT里SCORE的平均值。我在添加STUDENT，CLASS中的SCORE也跟着改变