Class 1 | Week 3 | DAT601

Todd has started the lesson with a short explanation on generalized entities which is looks like we will be revisiting at a later date, then he has started to explain to us two different problems that can be encountered in ER models; Fan traps & chasm traps.

The general take I am gathering is that linear built databases cant create errors in your database, generally a database would be build in a more circular fashion.

Following this he has moved into discussing generalized & specialized entities, by modeling it with a picture on the board. The concept feels similar to the concept of OO programming with abstract classes and classes below that that then inherit the default attributes of the base class as well as adding their own unique requirements.

His diagram on the board is demonstrating the way to model this form of "inheritance" in an ER model diagram, the highest level entity is called the "generalization" and the entities below it are called "specializations". Within this hierarchy there can be multiple levels of inheritance, this way its possible to model for example; different types of staff members with an educational institute.

This type of notation can also include participation rules.






Class notes

entities
relationships
attributes
cardinality
participation

generalized entity

Problems with ER models
    connection traps

Fan out & chasm

    fan trap
two entities have a 1 to many relationship that fan out from this entity, but the two entities should have a direct               relationship between them to provide the necessary information

    chasm trap

A chasm works in the opposite way but is solved in the same way (adding a joining relationship)


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