What is the difference between the many-to-many relationship between the three geometries and the three-to-many relationship between the two

Definition:

When there are many-to-many connections between an entity set and other entity sets between more than two entity sets, and there is no connection between other entity sets, this association is called the many-to-many association between multiple entity sets.

 
For example, there are three entity sets: suppliers, projects, and parts. One supplier can supply multiple parts for multiple projects. Each project can use parts from multiple suppliers; each part can be provided by different suppliers.
 

But there is no connection between the project and the part.

 
. Therefore, the supplier, project, and parts are multiple-to-multiple links. Supplier <---> Project: many-to-many, supplier <---> parts: many-to-many, the project has no clear contact with the parts.
 

 
Certificate ----------------------------------------------------------------------------------------------------------------------------------------
 

One question can help you better understand:

GiveEIRFigure, requires three solid types, and3There are many-to-many relationships between physical entities.3Is the multi-to-many relationship between physical entities equivalent to the three multi-to-many connections between the three physical entities? Why?

A:

 

3Multiple-to-multiple connections and3Real-size3Multiple-to-multiple connectionsIs not equivalentBecause they have different semantics.3Shows three multiple-to-multiple connections between two real-size instances.

 

The first question is the relationship between Middle School Students and courses and classrooms (Class), While the classroom has nothing to do with the course. Each of the second two has its own relationship.