Interesting tree/hierarchical data structure problem

Question

Colleges have different ways of organizing their departments. Some schools go School -> Term -> Department. Others have steps in between, with the longe

Answer 1

-- Enforcing a taxonomy by self-referential (recursive) tables.
-- Both the classes and the instances have a recursive structure.
-- The taxonomy is enforced mostly based on constraints on the classes,
-- the instances only need to check that {their_class , parents_class} 
-- form a valid pair.
--
DROP schema school CASCADE;
CREATE schema school;

CREATE TABLE school.category
  ( id INTEGER NOT NULL PRIMARY KEY
  , category_name VARCHAR
  );
INSERT INTO school.category(id, category_name) VALUES
  ( 1, 'School' )
  , ( 2, 'Sub_campus' )
  , ( 3, 'Program' )
  , ( 4, 'Term' )
  , ( 5, 'Division' )
  , ( 6, 'Department' )
  ;

-- This table contains a list of all allowable {child->parent} pairs.
-- As a convention, the "roots" of the trees point to themselves.
-- (this also avoids a NULL FK)
CREATE TABLE school.category_valid_parent
  ( category_id INTEGER NOT NULL REFERENCES school.category (id)
  , parent_category_id INTEGER NOT NULL REFERENCES school.category (id)
  );
ALTER TABLE school.category_valid_parent
  ADD PRIMARY KEY (category_id, parent_category_id)
  ;

INSERT INTO school.category_valid_parent(category_id, parent_category_id)
  VALUES
  ( 1,1) -- school -> school
  , (2,1) -- subcampus -> school
  , (3,1) -- program -> school
  , (3,2) -- program -> subcampus
  , (4,1) -- term -> school
  , (4,2) -- term -> subcampus
  , (4,3) -- term -> program
  , (5,4) -- division --> term
  , (6,4) -- department --> term
  , (6,5) -- department --> division
  ;

CREATE TABLE school.instance
  ( id INTEGER NOT NULL PRIMARY KEY
  , category_id INTEGER NOT NULL REFERENCES school.category (id)
  , parent_id INTEGER NOT NULL REFERENCES school.instance (id)
  -- NOTE: parent_category_id is logically redundant
  -- , but needed to maintain the constraint
  -- (without referencing a third table)
  , parent_category_id INTEGER NOT NULL REFERENCES school.category (id)
  , instance_name VARCHAR
  );      -- Forbid illegal combinations of {parent_id, parent_category_id}
ALTER TABLE school.instance ADD CONSTRAINT valid_cat UNIQUE (id,category_id);
ALTER TABLE school.instance
  ADD FOREIGN KEY (parent_id, parent_category_id)
      REFERENCES school.instance(id, category_id);
  ;
  -- Forbid illegal combinations of {category_id, parent_category_id}
ALTER TABLE school.instance
  ADD FOREIGN KEY (category_id, parent_category_id) 
      REFERENCES school.category_valid_parent(category_id, parent_category_id);
  ;

INSERT INTO school.instance(id, category_id
    , parent_id, parent_category_id
    , instance_name) VALUES
  -- Zulo
  (1,1,1,1, 'University of Utrecht' )
  , (2,2,1,1, 'Uithof' )
  , (3,3,2,2, 'Life sciences' )
  , (4,4,3,3, 'Bacherlor' )
  , (5,5,4,4, 'Biology' )
  , (6,6,5,5, 'Evolutionary Biology' )
  , (7,6,5,5, 'Botany' )
  -- Nulo
  , (11,1,11,1, 'Hogeschool Utrecht' )
  , (12,4,11,1, 'Journalistiek' )
  , (13,6,12,4, 'Begrijpend Lezen' )
  , (14,6,12,4, 'Typvaardigheid' )
  ;

  -- try to insert an invalid instance
INSERT INTO school.instance(id, category_id
    , parent_id, parent_category_id
    , instance_name) VALUES
  ( 15, 6, 3,3, 'Procreation' );

WITH RECURSIVE re AS (
  SELECT i0.parent_id AS pa_id
  , i0.parent_category_id AS pa_cat
  , i0.id AS my_id
  , i0.category_id AS my_cat
  FROM school.instance i0
  WHERE i0.parent_id = i0.id
  UNION
  SELECT i1.parent_id AS pa_id
  , i1.parent_category_id AS pa_cat
  , i1.id AS my_id
  , i1.category_id AS my_cat
  FROM school.instance i1
  , re
  WHERE re.my_id = i1.parent_id
  )
SELECT re.*
  , ca.category_name
  , ins.instance_name
  FROM re
  JOIN school.category ca ON (re.my_cat = ca.id)
  JOIN school.instance ins ON (re.my_id = ins.id)
  -- WHERE re.my_id = 14
  ;

The output:

INSERT 0 11
ERROR:  insert or update on table "instance" violates foreign key constraint "instance_category_id_fkey1"
DETAIL:  Key (category_id, parent_category_id)=(6, 3) is not present in table "category_valid_parent".
 pa_id | pa_cat | my_id | my_cat | category_name |     instance_name 
-------+--------+-------+--------+---------------+-----------------------
     1 |      1 |     1 |      1 | School        | University of Utrecht
    11 |      1 |    11 |      1 | School        | Hogeschool Utrecht
     1 |      1 |     2 |      2 | Sub_campus    | Uithof
    11 |      1 |    12 |      4 | Term          | Journalistiek
     2 |      2 |     3 |      3 | Program       | Life sciences
    12 |      4 |    13 |      6 | Department    | Begrijpend Lezen
    12 |      4 |    14 |      6 | Department    | Typvaardigheid
     3 |      3 |     4 |      4 | Term          | Bacherlor
     4 |      4 |     5 |      5 | Division      | Biology
     5 |      5 |     6 |      6 | Department    | Evolutionary Biology
     5 |      5 |     7 |      6 | Department    | Botany
(11 rows)

BTW: I left out the attributes. I propose they could be hooked to the relevant categories by means of a EAV type of data model.