查找未注册任何课程的学生姓名 - 学生、教师、课程、课程、已注册

根据下面的数据库，使用关系代数投影出未注册课程的学生的姓名。

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)

我可以通过以下方式获取所有未注册课程的学生的数量：

π snum Students - π snum Enrolled

但是我如何用我找到的 snums 来投影学生的名字呢？

每个基表都包含构成 true 的行主张（声明）来自一些（特征）谓词（按列参数化的语句模板）。设计者给出了谓词。用户不断更新表格。

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

每个查询结果都包含根据某个谓词得出正确命题的行。关系表达式的谓词由其参数表达式的谓词组合而成，具体取决于其谓词非终结符。 DBMS 评估结果。

/* 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两侧具有相同的列给出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

您可以从中投影出您不需要的任何列。

（请注意，我们不需要知道查询的约束。）

银行场景的关系代数 https://stackoverflow.com/a/24425914/3404097
从英文描述形成关系代数查询 https://stackoverflow.com/a/43318188/3404097
是否有任何经验法则可以根据人类可读的描述构建 SQL 查询？ https://stackoverflow.com/a/33952141/3404097