关系代数和混淆一个复杂的例子 [英] Relational Algebra and Confusing one Complex Examples

问题描述

我们有3个关系：

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

找到这些学生参加CS部门中所有课程的学生数量。

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

为什么（第4行）是此查询的答案？

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

任何人都可以说（第1行）到（第3行）。我在DB中的Solved-Ex中读到了。

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

推荐答案

第四行的另一种说法是：

Another way to phrase the fourth line is:

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

括号内的集合包含所有未采用的学生+ CS课程组合。从所有学生中减去这个，你得到的所有CS考试的学生。

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.

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