Robin Le Couteur NMIT Blog - 2018

Friday, March 23, 2018 Class Reflection Transforming Conceptual to Logical Many to Many Relationships Many to many relationships do not exist in a database, so there must be a way to model that sort of relationship. Many to many can exist in a conceptual model but you have to remove direct many to many relationships in a logical model. An example of how this might work is a many to many relationship between student and course.  Many students can be part of many courses and many courses can have many students. What you would do is add a table in between the two tables called StudentCourse. It will contain the courseID and Student ID and additional info as required. The 2 tables will each have a one to many relationship to the StudentCourse table Conceptual example Logical example As the example shows, a connecting table is added and the many to many relationship is removed. This model allows many students to be related to many courses and vice versa without t...

Friday, March 23, 2018 ER Modelling Continued pt.2 Today we covered more ER modelling concepts Enhanced ER Modelling We had a look at EER modelling, here is some of the rules and how they can be used: Union Types Union Type or Category The Idea of unions is to show how entities can have categories and types. U - Categorical or union D - The relationship between sub class and super class is denoted with the D  symbol. Sub class is a type of the generalized super class. Here is an example of how a Shape super class has different   types or sub classes. The Super class can be one of the subclasses O - Overlap. Can be one or many of the different options

Friday, March 16, 2018 ER Modelling Continued pt.1 Today we covered more stuff regarding ER modelling Firstly, we covered stuff from last lesson as a refresher before we moved on the the next subject Weak and Strong Entities Here we looked at different entity types: weak and strong entities.  Basically, a weak entity is an entity that doesn't have a key attribute by itself, and a strong entity is a normal entity with a key. The weak entity cannot exist without the strong entity it belongs to.  An example of this is if there was the entities Order and Order Detail. An Order Detail cannot exist unless it exists in an Order because it is partially identified by an order. Basically a recept can have a list of details but you can't have the details without the recept. Another example is: if there is a car, it can have ownership details, but you can't have ownership details if there is no car to own. Weak entities can be thought of as complex attributes. ...

Tuesday, March 13, 2018 ER Diagrams In todays class, we covered a number of different points regarding ERD's. Some of the stuff I remember from DAT 5, but there was some new stuff that I didn't know. We covered concepts like what an entity is vs an entity type or entity set. Some info I already knew that we covered included attribute types, so it was good just as a refresher. The concept of derived attributes and stored attributes was new though but easy to understand.  The subject of keys was quite interesting though because I had only dealt with composite, foreign, primary, and candidate keys. I now know that foreign keys should be ignored in the conceptual ERD, and I understand superkeys. Superkeys are used to uniquely identify a tuple(which is basically a row in a database).  Lastly, we covered relationships and how you can have binary, ternary, and more relationships between different entity types. You can also have one entity type in a relatio...