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Query Rewriting (cont)

But not all subqueries can be converted to join, e.g. 

select e.sid as student_id, e.cid as course_id
from   Enrolments e
where  e.sid = (select max(id) from Students)

has to be evaluated as

Val = max[id]Students

Res = π(sid,cid)(σsid=ValEnrolments)