Home » Professor NL Sarda Discusses Data Modeling – DFD, Function Decomp Part 2 (Transcript)

# Professor NL Sarda Discusses Data Modeling – DFD, Function Decomp Part 2 (Transcript)

• Motivation

Let us continue with the data modeling topic. So far we have seen the basic features of ER model, where to use it, what advantages it offers. We studied the concept of entity attribute and the key attributes. We also looked at the relationship concept. And then we looked at a simple diagramming notation through which you can show some entities, some relationships among them. This is what we call an instance diagram. And it is always good to draw an instance diagram to illustrate the understanding, or to illustrate the concepts we are modeling in our diagram. So let us proceed further.

We were talking about relationship cardinality as an additional constraint on a relationship. It tries to characterize the relationship more precisely by specifying how many entities of one set may relate with another entity in another set as a part of the relationship. So primarily, by cardinality, we are indicating how many entities in terms of one or more are related to entities from another set in forming a relationship. This is a very useful concept for binary relationships where two entity types or two entity sets are related.

If you consider a relationship named R between two entity sets A and B, then what are the possible cardinalities that we can indicate? The first one is of type one-to-one. It means that one entity in A is associated with at most one entity in B. So here it’s a one-to-one relationship. One-to-many relationship is where one entity in A may be associated with zero or more entities in B. So when we go from A to B, we may meet many entities from B which are associated or connected through the relationship with the given entity in A. So one-to-many is also an indication that if we start traversing or navigating in the database, when we start from A we may reach multiple instances of entities in B. This is characterized as one-to-many relationship. Obviously in a given relationship like this, if you now go from B to A, then one entity in B you would reach at most one entity in A. So one-to-many and many-to-one are really the inverse of each other.

Then finally, you have the many-to-many relationship. In the many-to-many relationship, one entity in A may be associated with more than one entities in B and also the reverse. So here one entity in A might be reachable from many entities in B. And similarly, an entity in B may be reachable from many entities in A. So this is a many-to-many example of relationship cardinality.

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