study(sno,cno,score)查询至少选修了两门课程的学生学号：π1(σ(1=4Λ2!=5)(study x study))。select sno(学生的学号) from sc(学生选课表)group by sno having count（*）>1 select a.学号,b.姓名,a.cnt as 选修门数 from (select 学号，count(1) as as cnt from 选课表 group by...
有一教学数据库,包括学生(S),课程(C)和选修(SC)三个关系: S(SNO,SN,SA,S,SD)有属性学号、姓名、年龄、性别、系别; C(CNO,CN,TEACHER)有属性课程号、课程名、任课教师; SC(SNO,CNO,G)有属性学号、课程号、成绩。 用关系代数描述下列问题: (1)检索年龄大于23岁的男学生的学号和姓名; (2)检索至少选...