Find the names of students who are not enrolled in any course - Students, Faculty, Courses, Offerings, Enrolled

问题

Given the database below, project the names of the students who are not enrolled in a course using relational algebra.
Students(snum, sname, major, standing, age, gpa)
Faculty(fid, fname, deptid)
Courses(cnum, cname, course_level, credits)
Offerings(onum, cnum, day, starttime, endtime, room, max_occupancy, fid)  
Enrolled(snum, onum)

I can get the snum of all students not enrolled in a course with:

π snum Students - π snum Enrolled

But how do I project the sname of the student with the snums that I find?

回答1:

Every base table holds the rows that make a true proposition (statement) from some (characteristic) predicate (statement template parameterized by columns). The designer gives the predicates. The users keep the tables updated.

-- rows where student [snum] is named [sname] and has major [major] and ...
Students
-- rows where student [snum] is enrolled in offering [onum]
Enrolled

Every query result holds the rows that make a true proposition from some predicate. The predicate of a relation expression is combined from the predicates of its argument expressions depending on its predicate nonterminal. The DBMS evaluates the result.

/* rows where
    student [snum] is named [sname] and has major [major] and ...
AND student [snum] is enrolled in offering [onum]
*/
Student ⨝ Enrolled

AND gives NATURAL JOIN, ANDcondition gives RESTRICTcondition, EXISTScolumns gives PROJECTother columns. OR & AND NOT with the same columns on both sides give OR & MINUS. Etc.

/* rows where
THERE EXISTS sname, major, standing, age & gpa SUCH THAT
    student [snum] is named [sname] and has major [major] and ...
*/
π snum Students

/* rows where
THERE EXISTS onum SUCH THAT
    student [snum] is enrolled in offering [onum]
*/
π snum Enrolled

/* rows where
    (   THERE EXISTS sname, major, standing, age & gpa SUCH THAT
            student [snum] is named [sname] and has major [major] and ...
    AND NOT
        THERE EXISTS onum SUCH THAT
            student [snum] is enrolled in offering [onum]
    )
AND student [snum] is named [sname] and has major [major] and ...
*/
(π snum Students - π snum Enrolled) ⨝ Students

You can project out any columns that you don't want from that.

(Notice that we don't need to know constraints to query.)

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来源：https://stackoverflow.com/questions/49021254/find-the-names-of-students-who-are-not-enrolled-in-any-course-students-facult

标签

relational-algebra