考虑学生关系(姓名、性别、标记)，其中主键用下划线显示，与至少有一个男孩和一个女孩的班级中的学生有关。以下关系代数表达式产生什么? (注意：r 是重命名运算符)。 
join中的条件是“(sex =女性^ x =男性^标记≤ m)”
(一)分数最高的女学生名字
(B)比某男生分数高的女生名字
(C)分数不低于部分男生的女生姓名4)
(D)分数比所有男生都多的女生名字答案： (D)
解释：
The above relational algebra expression has two sub expressions.
The first one takes as input the Student relation (Student) and filters
out all the tuples where sex=female(r sex=female (Student))
and then projects their names (P name r sex=female (Student)).
So we get a new relation with names of all the female students.
The second one takes as input the Student relation and performs a rename
operation on one with attributes name, sex and marks renamed as n, x, m
respectively (r n, x, m(Student)) and then followed by a self-Cartesian
product on the Student relation. The condition (sex = female ^ m = male ^ marks ≤ m)
filters tuples with all female students from the first relation,
male students from the second relation and performs a Cartesian product where
marks of the female student is either less than or equal to a male student and
then projects their names. So we get a new relation with names of all female
students whose marks are lesser than at least one of the male student.
两个子表达式之间的差异运算符(-) 给出了所有
成绩超过全班男学生的女学生。
(从所有女学生的名字中删除所有分数在
至少是一名男学生)
这个解释是由Yashika Arora 提供的。这个问题的测验