把“SC”表中“叶平”老师教的课的成绩都更改为此课程的平均成绩

update SC set score=(select avg(SC_2.score) from SC SC_2 where SC_2.C#=SC.C# ) from Course,Teacher where Course.C#=SC.C# and Course.T#=Teacher.T# and Teacher.Tname=’叶平’);