join - Relational Algebra "Only Once" or "Exists once"

Question

So I have 2 relations

Student = {student id, name, address}
Course = {course no, title, subject}
Completed = {course no, student id, grade, semester}

and I want to display the name of students who have COMPLETED only one COURSE of "Physics" (Which is a subject)

I dont have problems joining the tables to get the data together, my problem is with how to get values that appear only once?

What I have so far

PICourse_no (σ Subject=′Physics′(COURSE))

That gets me all the course numbers that are Physics related

PIStudent_Id(σCourseNo= (PICourse_no (σ Subject=′Physics′(COURSE))))

And with that I think I'm getting the Id's of all the students who study a physics related course...but now here is my problem, how do I remove the students who have MORE than one physics related course?

Answer 1

"how do I remove the students who have MORE than one physics related course?"

That is done by the relational MINUS operator or one of its nephews (sometimes known as antijoin). As indicated in the comments, there are a major number of distinct sets of operators all called "relational algebra". You really have to look into which one you are supposed to be using.