Basic Database ER question

[RearAdmiral]

Can a many-to-many binary relationship have no intersection data?

I know that this type of relationship certainly can have intersection data, but is it possible for it to NOT have this data?

 

[JavaMomma]

I'm assuming this is a homework question...

Since I dont have the question in front of me... I am sort of guessing at what you trying to ask... with what you mean by intersection.

Lets say there is a many-to-many relationship between two tables say Students and Courses. Students can be registered in 0 or more courses and a course can have 0 or more students registered in it. So, is it possible for a student to not be registered in a class? or a class to not have any students registered in it?

ER Diagram (with many-to-many relationship):
Student >----< Course


Transposed to Server Model Diagram:
Student ----< Stu_Cou >---- Course

Stu_Cou is a bridge table... It represents what I think you are refering to as the intersection. So if you had a list of Students in the Student Table and a list of Courses in the Course table -- Wouldn't it be possible for there to be an empty Stu_Cou table -- like when no students are yet registered for any courses...

I've tried to explain things as best I can... and interpret your question and answer it in away that doesnt just give you a flat answer but instead give you an answer that explains it so that you hopefully can understand it and answer it for yourself.

If I've misinterpreted your question, let me know.

 

[ViviTheMage]

you mean a bridge?

Java shows it :

ER Diagram (with many-to-many relationship):
Student >----< Course


Transposed to Server Model Diagram: (AKA BRIDGE)
Student ----< Stu_Cou >---- Course

BTW : im in a Database Managment class, which I just had tonight for 3 hours, so ask away .

 

[RearAdmiral]

I posted the question just how it is stated...
The example in our book shows the many to many binary relationship as a Salesperson-Sells-product. There can't be zero salespersons or products as the entity set wouldn't even have to exist then. But it seems from that example that there would always have to be an intersection data of quantity.

 

[ViviTheMage]

Originally posted by: RearAdmiral
I posted the question just how it is stated...
The example in our book shows the many to many binary relationship as a Salesperson-Sells-product. There can't be zero salespersons or products as the entity set wouldn't even have to exist then. But it seems from that example that there would always have to be an intersection data of quantity.

what book? I may have the same one and I can go look real quick.

 

[RearAdmiral]

Originally posted by: ViviTheMage

Originally posted by: RearAdmiral
I posted the question just how it is stated...
The example in our book shows the many to many binary relationship as a Salesperson-Sells-product. There can't be zero salespersons or products as the entity set wouldn't even have to exist then. But it seems from that example that there would always have to be an intersection data of quantity.

what book? I may have the same one and I can go look real quick.

Fundamentals of database management systems. Mark Gillenson

 

[ViviTheMage]

Originally posted by: RearAdmiral

Originally posted by: ViviTheMage

Originally posted by: RearAdmiral
I posted the question just how it is stated...
The example in our book shows the many to many binary relationship as a Salesperson-Sells-product. There can't be zero salespersons or products as the entity set wouldn't even have to exist then. But it seems from that example that there would always have to be an intersection data of quantity.

what book? I may have the same one and I can go look real quick.

Fundamentals of database management systems. Mark Gillenson

dang, mines database systems , i think I may be a class above you.

 

[RearAdmiral]

Page 58, question 8
Where do you go to school, im at penn state...

 

[ViviTheMage]

Originally posted by: RearAdmiral
Page 58, question 8
Where do you go to school, im at penn state...

I just ran outside to take a look at it.

I think my books one up from yours. That one sounded so familiar, I thought it was it, but that was last semester, or last year, cant remember.

I'm at U of M - Twin Cities

 

[JavaMomma]

Originally posted by: ViviTheMage
you mean a bridge?

Java shows it :

ER Diagram (with many-to-many relationship):
Student >----< Course


Transposed to Server Model Diagram: (AKA BRIDGE)
Student ----< Stu_Cou >---- Course

BTW : im in a Database Managment class, which I just had tonight for 3 hours, so ask away .

Yes thats what I mean -- sorry that was mainly a reference to Oracle Designer.

 

[JavaMomma]

Originally posted by: RearAdmiral
I posted the question just how it is stated...
The example in our book shows the many to many binary relationship as a Salesperson-Sells-product. There can't be zero salespersons or products as the entity set wouldn't even have to exist then. But it seems from that example that there would always have to be an intersection data of quantity.

So you have two tables in your ER diagram?
SalesPersons with a many-to-many relationship with Products?

 

[RearAdmiral]

Originally posted by: JavaMomma

Originally posted by: RearAdmiral
I posted the question just how it is stated...
The example in our book shows the many to many binary relationship as a Salesperson-Sells-product. There can't be zero salespersons or products as the entity set wouldn't even have to exist then. But it seems from that example that there would always have to be an intersection data of quantity.

So you have two tables in your ER diagram?
SalesPersons with a many-to-many relationship with Products?

Right...it seems to me like it would always need a description/intersection data of "quantity"