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lec19
Let's say you are asked to model a business's data. The first question you may have is, where do I even begin. If you are in charge of this kind of project there are some things to consider:
- Scalability for future changes
- What kind of data you are dealing with
Design process
- Understand the problem at hand and needs of users
- Create conceptual design
Entity-Relationship (ER) Models
Used for conceptual design. There are 3 building blocks for an ER model:
- Entities
- Relationships
- Attributes
For the rest of this lecture we'll be using a book store as our working example to get through this concept.
Entity & Entity Set
Often with entities we're really referring to real world things which have properties of their own. An entity for a bookstore would just be something like a book, author, or a publisher. We would have to consider these things when modeling the pertinent data in regards to the business or organization, because we have to keep track of these things to ensure that the business runs smoothly.
If we keep track of things like the books in our store, then we might avoid accidently ordering too many of the same book, or running out of one specifically.
Relationships
A relationship is simply an association between entities. Furthermore entities, can participate in relationships by simply being related to some other entity.
Coming back to our exmample, books and publishers, are two entities whom participate in a relationship together. Likewise we can two books which may be a part of a long running series, which means they should be related. This would mean we have a book in a relationship with a book. Each book however, takes on a different role in the relationship; perhaps one book is the sequel to the other.
A more clear example might be that a book has an author which means the two must be related, therefore we may create associate some kind of relationship. Likewise the same book entity may participate in another relationship if appropriate, like with the publisher.
Mapping Cardinalities
One-to-One Mapping
Say we have to entity sets, where each entity in each set is related to one entity in the opposing set[ex. every office has 1 instructor]. A is in a relationship with B or, B is in a relationship wth A. Both explanations are fine and valid.
One-to-Many/Many-to-One
All the entities in one set are related to by at most one entity from the other set[A student can have at most one advisor]. The student set would by one-to-many while the advisor set would be many-to-one.
Many-to-Many
Each entity in a given set may have zero or more relationships with entities in an opposing entity set. Likewise the inverse is also true.