View source for Viewer:4/Logical

== Logical ==

[[File:Viewer-Relational.png]]

The logical tab shows the tables, columns, primary keys, foreign keys. They are not drawn by hand, simply derived and visualized from the CaseTalk Modeler files. Some remarks are in place to explain the diagram:

* Person is a regular table with two columns, both part of the primary key (hence the key icon).
* Student is a table which has a subtype relation towards the supertype Person. A curved short line is drawn to indicate this. Zero or more Students are a subtype of Person. And the Project Code refers to the Project table, indicated by the chain icon.
* At least one Student needs to exist to register Preferences. And Preferences may occur once or more.
* Preferences is so called weak table. Since the primary key consists of columns which are used in a a foreign key. In other words, the Preferences cannot be identified by itself, it requires the Student as well. Therefor the table is considered to be a weak entity type.
* The Project table can be populated without the existence of Preferences or Students. The ProjectCode in Student references a Project, yet it is not mandatory and not required to identify any Student. Therefor the foreign keys for Project are both considered to be weak relations.