How to convert sql to relational algebra?

[saltedfish]

Now I have a sql needed to be converted to relational algebra. There are two tables: student and sc.
student has sno, sname, sage.
sc has sno, cno, grade.
How to translate the sql into relational algebra:
select sname from student where sage > 20 or sno in (select sno from sc where grade > 90);

I know it can be converted to union opertaion, but that is too complicated for many sqls like this. Are there any other methods?
I have a plan of sql server, but I could not understand what it means?
Any help with this would be great.

[NeoPa]

I'm not sure what you're actually asking as I have no idea what SC might refer to as a table and I haven't come across the term Relational Algebra. That said you may be looking for something like :

SELECT   tS.SNo
       , [SName]
FROM     [Student] AS tS
         LEFT OUTER JOIN
         [SC]
  ON     tS.SNo = SC.SNo
WHERE   (tS.SAge > 20)
   OR   (MAX(SC.Grade) > 90)
GROUP BY tS.SNo

[saltedfish]

First thank you very much for you reply.
I have unload a picture about the relational algebra.
In fact, there are three tables: student, sc, course.
course has cno and cname, respectively representing the number and name of course. sc has sno, cno, grade, respectlively representing number of student, number of course and the score.
What I mean is this:

select sname 
      from student 
      where sage > 20 or 
            sno in (select sno from sc where grade > 90)

It is equal to:

     select sname from student where sage > 20
     union
     select sname from student 
       where sno in (select sno from sc where grade > 90)
 

Can it be expressed as like this:

    select sname 
    from student,(select sno from sc where grade > 90) ta
    where sage > 20 or student.sno = ta.sno
 

Maybe there has exists predicate:

   select sname from student
   where sage > 20 or
         sno in (select sno from sc where grade > 90) or
         exists (select * from course)
 

I don't care what the sqls means, but I only care how the sqls are translated to relational algebra.

[Stewart Ross]

Hi Saltedfish. What you ask is related to a course assignment and is beyond what we can assist you with. We normally deal with expressing queries in Structured Query Language form, not in reverse-engineering these back to an academic form of logic such as relational algebra (see http://en.wikipedia.org/wiki/Relational_algebra).

I studied predicate calculus many years ago, but even so I don't think this forum is the place for what you are asking. You are asking us to take a SQL-based implementation of relationships between tables and convert that back to its relational algebra equivalent.

Even if we were experts on this - and I for one am not - we cannot do your work for you when it comes to academic coursework.

-Stewart