1ãselect Sno,Sage from student where Sage<20; 2ãselect s.Sname,c.Cname,SC.grade from student s,course c, SC where s.Sno=SC.Sno and c.Cno=SC.Cno; 3ãselect ä½è .ä½è å§å,å¾ä¹¦.å¾ä¹¦åï¼å¾ä¹¦.åºç社 from å¾ä¹¦ï¼ä½è where å¾ä¹¦.ä½è ç¼å·=ä½è .ä½è ç¼å· and ä½è .ä½è ç¼å· in (select ä½è ç¼å· from ä½è where å¹´é¾<(select AVGï¼å¹´é¾ï¼ from ä½è ))