急！请大侠们指点，一道数据库的题。写出关系代数表达式~~非常感谢！

1.use database_name
select s.sno,c.credit from course c join elective e
on c.con=e.con join student s
on s.sno=e.sno
where c.con='c06'

2..use database_name
select s.sname,c.sname from course c join elective e
on c.con=e.con join student s
on s.sno=e.sno
where c.con='c06'

3.select s.sname,c.sname from course c join elective e
on c.con=e.con join student s
on s.sno=e.sno
where c.cname='ENGLISH'

4..select s.sno from course c join elective e
on c.con=e.con join student s
on s.sno=e.sno
where c.cno in('c02','c06')

5...select s.sno from course c left join elective e
on c.con=e.con join student s
on s.sno=e.sno
where c.cno in('c02','c06')

6...select s.sno from course c left join elective e
on c.con=e.con join student s
on s.sno=e.sno
where c.cno not in('c02','c06')

7.select sname from student

8.select s.sno from course c left join elective e
on c.con=e.con join student s
on s.sno=e.sno
where s.sno like['s08'%]
呵呵，就这些了

关系代数的符号不好表达呀
两者理解起来都一样的……
很简单的

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