= Relational Design =

This page documents the relational design for BEST Skopje Hub, based on !ERModel_v09. The design uses partial transformation from the ER model into a PostgreSQL relational schema. Strong entities are mapped directly to relations, weak entities include the primary keys of their owner entities, specializations are mapped with one relation for the supertype and one relation for each subtype, and M:N relationships are mapped as separate relations.

Some attribute names were adapted to SQL-friendly snake_case names. For example, the ER attribute `index` is represented as `student_index`, and the !EventEditions attributes `title`, `start`, and `end` are represented as `event_title`, `event_start_time`, and `event_end_time`.

== Descriptive representation of the relational schema ==

{{{
students(
    student_index, first_name, last_name, faculty, field_of_studies,
    year_of_studies, birthday, email, phone_number,
    PK(student_index)
)

members(
    student_index,
    PK(student_index),
    FK(student_index) -> students(student_index)
)

observers(
    student_index,
    PK(student_index),
    FK(student_index) -> members(student_index)
)

young_members(
    student_index, mentor_index,
    PK(student_index),
    FK(student_index) -> members(student_index),
    FK(mentor_index) -> full_members(student_index)
)

full_members(
    student_index,
    PK(student_index),
    FK(student_index) -> members(student_index)
)

alumni(
    student_index,
    PK(student_index),
    FK(student_index) -> members(student_index)
)

membership_stages(
    ms_name,
    PK(ms_name)
)

has_stage(
    student_index, ms_name, valid_from, valid_to,
    PK(student_index, ms_name, valid_from),
    FK(student_index) -> members(student_index),
    FK(ms_name) -> membership_stages(ms_name)
)

organizational_functions(
    of_title,
    PK(of_title)
)

board(
    of_title,
    PK(of_title),
    FK(of_title) -> organizational_functions(of_title)
)

non_board(
    of_title,
    PK(of_title),
    FK(of_title) -> organizational_functions(of_title)
)

is_organizational_function(
    student_index, of_title, mandate,
    PK(student_index, of_title, mandate),
    FK(student_index) -> full_members(student_index),
    FK(of_title) -> organizational_functions(of_title)
)

activity_types(
    at_name, at_description,
    PK(at_name)
)

activity_sessions(
    at_name, as_start_time, as_location,
    PK(at_name, as_start_time, as_location),
    FK(at_name) -> activity_types(at_name)
)

related_to(
    at_name, as_start_time, as_location, et_name, event_title, event_start_time,
    PK(at_name, as_start_time, as_location),
    FK(at_name, as_start_time, as_location) -> activity_sessions(at_name, as_start_time, as_location),
    FK(et_name, event_title, event_start_time) -> event_editions(et_name, event_title, event_start_time)
)

event_types(
    et_name, et_description,
    PK(et_name)
)

event_editions(
    et_name, event_title, event_start_time, event_end_time,
    PK(et_name, event_title, event_start_time),
    FK(et_name) -> event_types(et_name)
)

event_edition_locations(
    et_name, event_title, event_start_time, location,
    PK(et_name, event_title, event_start_time, location),
    FK(et_name, event_title, event_start_time) -> event_editions(et_name, event_title, event_start_time)
)

event_session_types(
    est_name, est_description, needed_no_volunteers,
    PK(est_name)
)

event_sessions(
    et_name, event_title, event_start_time, est_name, es_start_time, location, es_title,
    PK(et_name, event_title, event_start_time, est_name, es_start_time, location),
    FK(et_name, event_title, event_start_time) -> event_editions(et_name, event_title, event_start_time),
    FK(est_name) -> event_session_types(est_name)
)

core_team_functions(
    et_name, event_title, event_start_time, function_name,
    PK(et_name, event_title, event_start_time, function_name),
    FK(et_name, event_title, event_start_time) -> event_editions(et_name, event_title, event_start_time)
)

applications(
    application_id, student_index, et_name, event_title, event_start_time,
    function_name, motivational_letter, application_type, application_status,
    PK(application_id),
    FK(student_index) -> students(student_index),
    FK(et_name, event_title, event_start_time) -> event_editions(et_name, event_title, event_start_time),
    FK(et_name, event_title, event_start_time, function_name) -> core_team_functions(et_name, event_title, event_start_time, function_name)
)

participation(
    student_index, at_name, as_start_time, as_location,
    rsvp_status, rsvp_at, attendance_status,
    PK(student_index, at_name, as_start_time, as_location),
    FK(student_index) -> students(student_index),
    FK(at_name, as_start_time, as_location) -> activity_sessions(at_name, as_start_time, as_location)
)

announces_absence(
    student_index, at_name, as_start_time, as_location, reason, announced_at,
    PK(student_index, at_name, as_start_time, as_location),
    FK(student_index) -> members(student_index),
    FK(at_name, as_start_time, as_location) -> activity_sessions(at_name, as_start_time, as_location)
)

responsible_for(
    student_index, at_name, as_start_time, as_location,
    PK(student_index, at_name, as_start_time, as_location),
    FK(student_index) -> members(student_index),
    FK(at_name, as_start_time, as_location) -> activity_sessions(at_name, as_start_time, as_location)
)

volunteers(
    student_index, et_name, event_title, event_start_time,
    est_name, es_start_time, location, volunteer_role,
    PK(student_index, et_name, event_title, event_start_time, est_name, es_start_time, location),
    FK(student_index) -> members(student_index),
    FK(et_name, event_title, event_start_time, est_name, es_start_time, location) -> event_sessions(et_name, event_title, event_start_time, est_name, es_start_time, location)
)

companies(
    c_id, c_name, naframa_reference,
    PK(c_id)
)

cooperation(
    c_id, et_name, event_title, event_start_time, cooperation_type,
    PK(c_id, et_name, event_title, event_start_time),
    FK(c_id) -> companies(c_id),
    FK(et_name, event_title, event_start_time) -> event_editions(et_name, event_title, event_start_time)
)
}}}

In the physical PostgreSQL schema, `application_id` is implemented as an automatically generated identity value. It is still listed as the primary key of `applications`, but it does not have to be manually inserted in the data loading script.

The `applications` relation is used only when there is an actual application process. Public attendance for events such as Job Fair, and open registration-style attendance for events such as beBESTie, is not represented as an application in this phase. For Hackathon and BEST Course in Summer participation applications, the `motivational_letter` attribute represents the most important motivation/answer kept from an external application form.

The specialization of students into members and then into observers, young members, full members, and alumni is represented through subtype tables whose primary keys are also foreign keys. `young_members` also contains `mentor_index`, which represents the mentorship relationship directly as a foreign key to `full_members`.

The weak entities from the ER model are represented using composite primary keys. `activity_sessions` includes the key of `activity_types`. `event_editions` includes the key of `event_types`. `event_sessions` includes the composite key of `event_editions`, the key of `event_session_types`, and its partial key attributes `es_start_time` and `location`. `core_team_functions` includes the composite key of `event_editions` and its partial key `function_name`.

The multivalued `locations` attribute of !EventEditions is transformed into the separate relation `event_edition_locations`.

The relationships `has_stage`, `participation`, `announces_absence`, `responsible_for`, `volunteers`, `cooperation`, `related_to`, and `is` are represented as relations because they either have M:N cardinality, their own attributes, or are clearer without nullable foreign-key columns. The relationship `related_to` is represented separately because only some activity sessions are connected to event editions. The relationship `applies_with` is represented with the foreign key `student_index` in `applications`, because each application belongs to one student. The N:1 relationships `for_function`, `for_participation`, and `mentorship` are represented directly with foreign keys on the N-side relation.

== DDL script for creating the database schema and objects ==

The DDL script recreates the official `project` schema and all database objects. It first drops the existing `project` schema if it exists, then creates the schema, tables, primary keys, foreign keys, and checks.

[attachment:schema_creation.sql schema_creation.sql]

== DML script for filling tables with data ==

The DML script recreates realistic sample data for all relations. It clears the current contents of the tables and inserts connected sample data for students, members, membership stages, activity sessions, event-related activity links, event editions, event sessions, applications, companies, volunteering, mentorship, attendance/RSVP, announced absences, and organizational functions.

[attachment:data_load.sql data_load.sql]

== Relational diagram ==

The relational diagram was exported from DBeaver after running `schema_creation.sql` in the assigned PostgreSQL database and opening the `project` schema diagram. The diagram uses crow-feet notation.

[[Image(relational_schema.png, width=100%)]]

== AI Use ==

AI was used during this phase as a consultation, drafting, and checking tool while preparing the relational design from the already completed !ERModel_v09. The AI-assisted parts were reviewed against the existing ER model and project requirements before being included.

Full AI usage documentation: [wiki:RelationalDesignAIUsage RelationalDesignAIUsage].