databaserelational-databaserelational-algebrarelational-division

find student numbers of students who take all courses in department 'CS'

students (sid, sname)
courses (cid, cname, dept)
take (sid, cid, grade)

We want to find student numbers of students who take all courses in department 'CS'.

Why is line 4 the answer?

How does it differ from lines 1 through 3?

I read this in Solved-Ex in DB.

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Solution

  • Another way to phrase the fourth line is:

    all students
except
(
    all combinations of students and CS courses
    except
    all taken courses
)

    The set between brackets contains all student + CS course combinations that were not taken. Subtract this from all students, and you get the students that did take all CS exams.