== '''Entity-Relationship Model v01'''

== Diagram

[[Image(ERModel_v1.3.3.drawio.png)]]

== Data Requirements

=== '''University'''
This entity stores information about universities. It defines which faculties belong to which university and tracks affiliations with professors.

'''Candidate keys:''' id (primary key).

'''Attributes:'''

'''id''' – serial (integer), required, unique

'''name''' – varchar(255), required

'''location''' – varchar(100), optional

'''isprivate''' – boolean, required (mapped to is_private on the diagram)

=== '''Faculty'''
Represents a faculty within a university. It organizes students, professors, and subjects.

'''Candidate keys:''' id (primary key).

'''Attributes:'''

'''id''' – serial (integer), required, unique

'''name''' – varchar(255), required

'''location''' – varchar(100), optional

'''study_field''' – study_field_enum, required

=== '''Professor'''
This entity stores professor details.

'''Candidate keys:''' id (primary key).

'''Attributes:'''

'''id''' – serial (integer), required, unique

'''name''' – varchar(255), required

'''surname''' – varchar(255), optional

'''age''' – integer, optional, range 18–80

=== '''Student'''
This entity stores student details.

'''Candidate keys:''' id (primary key), studentindex (unique key).

'''Attributes:'''

'''id''' – serial (integer), required, unique

'''name''' – varchar(255), required

'''surname''' – varchar(255), optional

'''location''' – varchar(100), optional

'''studentindex''' – integer, required, unique (mapped to Index on the diagram)

=== '''Subject'''
This entity stores the courses or subjects offered by a faculty.

'''Candidate keys:''' id (primary key).

'''Attributes:'''

'''id''' – serial (integer), required, unique

'''name''' – varchar(255), required

'''semester''' – integer, optional, range 1–8

'''credits''' – integer, optional, range 1–10

=== '''Student_Subject (Weak Entity)'''
This entity represents a weak entity that depends on both Student and Subject for its identity. It tracks a specific student enrollment in a subject (course instance).

'''Discriminator (Partial Key):''' ss_id.

'''Composite Primary Key:''' (student_id, subject_id, ss_id)

'''Attributes:'''

'''ss_id''' – integer, required. Acts as the discriminator (partial key/dashed underline) for the weak entity. It uniquely identifies multiple enrollments of the same student in the same subject.

'''enrollment_date''' – date, required, defaults to CURRENT_DATE (mapped to Enrollment_Date).

'''status''' – varchar(20), optional, defaults to 'Enrolled'.

'''final_grade''' – integer, optional (mapped to Final_Grade).

'''absences_count''' – integer, optional, defaults to 0 (mapped to Abesense_Count on the diagram).

== Relationships

=== '''part_of'''
Binary 1:N relationship between University (1) and Faculty (N). Indicates which university a faculty belongs to.

=== '''employs'''
Binary 1:N relationship between Faculty (1) and Professor (N). Tracks which faculty employs a professor.

=== '''enrolled_in'''
Binary 1:N relationship between Faculty (1) and Student (N). Tracks which faculty a student is enrolled in.

=== '''offers'''
Binary 1:N relationship between Faculty (1) and Subject (N). Defines which subjects are offered by a faculty.

=== '''affiliated_with'''
Binary M:N relationship between University and Professor. Allows a professor to be affiliated with multiple universities.

=== '''Advises'''
Binary M:N relationship between Professor and Student tracking mentorship.

'''Attributes:'''

'''start_date''' – date, required

'''end_date''' – date, optional

=== '''Teach'''
Binary M:N relationship between Professor and Subject. Indicates which subjects a professor can teach.

=== '''enrolls'''
Identifying 1:N relationship between Student (1) and weak entity Student_Subject (N).

=== '''has'''
Identifying 1:N relationship between Subject (1) and weak entity Student_Subject (N).

=== '''assignTo'''
Regular 1:N relationship between Professor (1) and weak entity Student_Subject (N). Indicates which professor is assigned to a specific course instance enrollment.

== Entity-Relationship Model History

v1: First version of the conceptual ER model including University, Faculty, Professor, Student, Subject, Student_Subject weak entity, and precise entity relationships.

v1.1: Corrected conceptual ER representations: removed all foreign keys from entity attributes, correctly specified weak entity discriminator and identifying relationships, converted bridge tables (Affiliated, Advises) into relationship types with attributes.
v1.2: Removed underline from start_date in Advises relationship (since relationships cannot have primary key attributes), renamed duplicate enrolled_in relationship to enrolls, and ensured identifying relationships for Student_Subject are formally represented.