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Join Example (cont)

List names of students in all subjects, arranged by subject.

SQL query to provide this information: 

select E.subj, S.name
from   Student S, Enrolled E
where  S.id = E.stude
order  by E.subj, S.name;

And its relational algebra equivalent:

Sort[subj] ( Project[subj,name] ( Join[id=stude](Student,Enrolled) ) )


To simplify formulae, we denote Student by S and Enrolled by E