Relational Algebra and Confusing one Complex Examples

问题

We have 3-Relation:

Students(sid, sname)
Courses(cid, cname, dept)
take(sid, cid, grade)

We want to find student numbers of students whose these students take all courses that present in 'CS' department.

why (line 4) is the answer of this query ?

anyone could say differ from (line 1) to (line 3). I read this in Solved-Ex in DB.

回答1:

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.

来源：https://stackoverflow.com/questions/28942035/relational-algebra-and-confusing-one-complex-examples