用sql语句查询选修了2门以上课程的学生学号和平均成绩。

表结构:
定义表结构的SQL语句如下：

CREATE TABLE student (

sno varchar(6) NOT NULL ,

sname varchar(10) ,

sex char(2) ,

nation char(2) ,

pnum char(18) ,
birth date ,

phone char(11) ,

dept varchar(20) ,

PRIMARY KEY (sno)

) ;

CREATE TABLE course (

cno varchar(6) NOT NULL,

cname varchar(20) ,

credit int(11) ,

attribute varchar(10) ,

PRIMARY KEY (cno)

) ;

CREATE TABLE score (

sno varchar(6) NOT NULL,

cno varchar(6) NOT NULL,

term varchar(15),

grade int(11),

PRIMARY KEY (sno,cno)

) ;

select  t.sno,avg(t.grade) avg_grade
  from score t 
    join course t1 on t.cno=t1.cno and t1.attribute='选修'
  group by t.sno
    having count(*)>2