woensdag 6 maart 2019

Fact Oriented Modeling constraints (part II)


In my previous blogpost about FOM, I've discussed the basics of FOM. In this blogpost I would like to elaborate on constraints in FOM. Constraints are important in data modelling because they limit the degrees of freedom (of the data) in the model. In this blogpost I'll describe the different constraints that can be applied to the FOM data models.

As said before, there are different constraints possible in FOM :
  • Value Constraints.
  • Uniqueness Constraints.
  • Totality Constraints.
  • Subset Constraints.
  • Equality Constraints.
  • Mutually Exclusion Constraints.
  • Number constraints.

Value Constraints
Valueconstraints are limitations of certain labeltypes, for instance a sequencenumber can only exist of 1, 2 or 3. You can add this in CaseTalk. 

This result in the following change in the diagram. It is now visible that sequencenumber can only exist of 1, 2 or 3.

Uniqueness Constraints
Yet another constraint is uniqueness, meaning that values are unique in a population. You can do this with the menu option Set Unique Constraint in CaseTalk.

This results in the following constraint on Student, saying that there can be only one student with a firstname and a lastname.

There are also other combinations possible, for instance :

Here you say that Peter Jansen is unique and this means that Peter cannot live in two places at the same time. The other way around: In Nijmegen it is possible that two students live here. So, there is not an unique constraint needed.

Another example is presented below. Here you can see that there are two unique constraints on separate columns, meaning that a student is unique and internship is unique. This means that a student can only do one internship and an internship can only be done by one student.

Here is a more complicated uniqueness constraint. In this diagram there 3 roles and two uniqueness constraints:
  • Student and internship.
  • Student and sequencenumber.

The combination of student and internship is unique. That makes sense. You can not apply twice for the same internship. The other uniqueness constraint is student and sequencenumber is also logical. A student can not apply for an internship with the same sequencenumber. The student should make an order of internships.

After determining the uniqueness constraints a couple of tests are necessary and they are organized as follows:
  • Elementary test
    • n-1 rule test
    • n rule test
    • Projection/Join test
  • Nominalisation test

Totality Constraints.
Totality constraints are constraints that says that every tuple from a role of a (nominalised) facttype should be present in a involved role. For instance: "Every student should have a place to live" meaning that when a student is known, the place is also known. If a place is not known, this is not possible in the semantics. This is not the fact. The fact is that a student has a place. So, you identify is with a totality constraint. How to set this up in CaseTalk?

I had to figure this one out but I managed to make a totality constraint. First you have to press the <CTRL> button and then click on the role you want to put a Totality constraint on.

Now when you are done, The following diagram has Totality Constraints.

Setting Totality Constraints should be handled with care, because when generating a data model with a totalityconstraints can result in a NOT NULL column.

Subset Constraints.
Subset constraints are constraints that says something about subsetting of particular roles. This states that the set of roles is a subset of another set of roles.The values of oneset of roles should be present in the other set of roles.

Let's experiment this with CASE Talk. First press on the subset constraint Icon in the top left in diagram pane.

This will show the following window and here you can enter the subset constraint with the From and To part.

And here is the result of the subset constraint.

The plotted size is a bit huge in contrast with the diagram. You can change that in the diagram with Style and Options.

And here you set the font size.

Equality Constraints.
An equality constraint is needed when two set of roles are equal. This is entered as two subset constraint and is only changed in the From and To.

Mutually exclusion constraints
Mutually exclusive constraints are constraints that are exclusive between two roles. These roles cannot have a common population. For instance, You're married or divorced. You can not be both at the same time.

Number constraints
Number constraints are constraints that limit the values in a role. For instance a student can only enter three preferences for a internship.

Final thoughts

Setting the constraints on the FOM datamodel with CaseTalk will limit the degrees of freedom in your information model and help you structure the information and ultimately a physical model.


Hennie de Nooijer

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