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！

1use database_name
select ssno,ccredit from course c join elective e
on ccon=econ join student s
on ssno=esno
where ccon='c06'
2use database_name
select ssname,csname from course c join elective e
on ccon=econ join student s
on ssno=esno
where ccon='c06'
3select ssname,csname from course c join elective e
on ccon=econ join student s
on ssno=esno
where ccname='ENGLISH'
4select ssno from course c join elective e
on ccon=econ join student s
on ssno=esno
where ccno in('c02','c06')
5select ssno from course c left join elective e
on ccon=econ join student s
on ssno=esno
where ccno in('c02','c06')
6select ssno from course c left join elective e
on ccon=econ join student s
on ssno=esno
where ccno not in('c02','c06')
7select sname from student
8select ssno from course c left join elective e
on ccon=econ join student s
on ssno=esno
where ssno like['s08'%]
呵呵，就这些了

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