问题描述
请考虑以下关系数据库方案：
Consider the following relational database schemes:
COURSES (Cno,name)
PRE-REQ(Cno, pre-Cno)
COMPLETED (student_no, Cno)
课程提供数字和名称所有可用的课程。
COURSES gives the number and name of all the available courses.
PRE-REQ提供有关哪些课程是给定课程的前提条件的信息。
PRE-REQ gives the information about which courses are pre-requisites for a given course.
COMPLETED表示学生完成了哪些课程
COMPLETED indicates what courses have been completed by students
使用关系代数表达以下内容：
Express the following using relational algebra:
列表学生_no = 2310的学生已完成所有课程的所有先决条件。
List all the courses for which a student with student_no = 2310 has completed all the pre-requisites.
该问题可以通过SQL查询解决，但无法导出等效的关系代数。
the question can be solved by SQL query but unable to derive equivalent relational algebra.
推荐答案
这可能是正确的答案
Π（PRE-REQ）-Π（PRE-REQ-（已完成⋈ PRE-REQ） ）
Π(PRE-REQ) - Π( PRE-REQ - (COMPLETED ⋈ PRE-REQ ))
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