Re: Inheritance in PostgreSQL

[Ron <ronljohnsonjr@xxxxxxxxx>]

    On 10/17/23 16:42, Luis Bruno wrote:

Hello, I'm in the process of developing a basic database structure that utilizes inheritance as part of a test for my work. The database consists of two tables: ‘PERSON' and ‘CUSTOMER' with the ‘PERSON' table serving as the parent table for ‘CUSTOMER' .

Initially, I defined the 'CREATE TABLE' statement as follows:

CREATE TABLE PERSON (
    id SERIAL PRIMARY KEY,
    name VARCHAR(255) NOT NULL,
    dob DATE
);

CREATE TABLE CUSTOMER (
    registration_date DATE NOT NULL,
    contact VARCHAR(255)
) INHERITS (person);

INSERT INTO PERSON VALUES (1, 'Fulano', '1965-06-07');
INSERT INTO CUSTOMER VALUES (2, 'Beltrano', '1980-10-07', '2023-10-10', '5561999999999');

With these ‘INSERTS’, we have three records, as expected:

The problem occurs when we try add the ‘Fulano’ as a customer:

INSERT INTO CUSTOMER (id, name, dob, registration_date, contact)
SELECT id, name, dob, '2023-10-17', 'contact@xxxxxxxxxxx'
FROM person
WHERE id = 1;

The 'CUSTOMER' table look like this:

However, this issue arises in the 'PERSON' table:

The primary key is duplicated when I attempted to add 'Fulano' as a customer.

After that, I attempted a slightly different approach in creating the ‘CUSTOMER’ table, as I'll show below:

CREATE TABLE customer (
  "id" int4 NOT NULL PRIMARY KEY DEFAULT nextval('person_id_seq'::regclass),
  name VARCHAR(255) NOT NULL,
  dob DATE,
  registration_date DATE,
  contact varchar(255)
) INHERITS (person);

But, when I run the same ‘INSERTS’ above, the same problem occurs with the ‘PERSON’ table:

I would like to know where I might be going wrong with these simple queries, and reinforce that my main question is: how to create a record for a ‘CUSTOMER’ who already exists in the ‘PERSON’ table?

A question that arose was to see that in the 'PERSON' table, there was a duplication of the record with the same 'id', considering that 'id' is a primary key.

I'm particularly interested in the advantages of the inheritance concept in PostgreSQL, considering that it can be easily applied to my business rules. I'd also like to know if inheritance is commonly used. Any insights and recommendations would be appreciated. Thank you.

    Data Normalization was "invented" to eliminate this problem (and
    many others).
    
    CREATE TABLE person (
          id BIGINT PRIMARY KEY GENERATED ALWAYS AS IDENTITY,
          name TEXT,
          dob DATE );
      
      CREATE TABLE customer (
          id BIGINT PRIMARY KEY GENERATED ALWAYS AS IDENTITY,
          person_id BIGINT REFERENCES person(id),
          registration_date DATE NOT NULL,
          contact_info TEXT);
      
      
      foo=# INSERT INTO person (name, dob) VALUES ('Fulano',
      '1965-06-07')
      foo-#     RETURNING id;
       id 
      ----
        1
      (1 row)
      
      INSERT 0 1
      foo=# 
      foo=# INSERT INTO customer (person_id, registration_date,
      contact_info)
      foo-# values (1, '2023-10-10', '867-5309');
      INSERT 0 1
      foo=# 
      foo=# INSERT INTO person (name, dob) VALUES ('Beltrano',
      '1980-10-07')
          RETURNING id;
       id 
      ----
        2
      (1 row)
      
      INSERT 0 1
      foo=# 
      foo=# INSERT INTO customer (person_id, registration_date,
      contact_info)
      values (2, '2023-10-12', '555-1212');
      INSERT 0 1
      foo=# 
      foo=# SELECT p.*, c.*
      foo-# FROM person p, customer c
      foo-# WHERE p.id = c.person_id;
       id |   name   |    dob     | id | person_id | registration_date |
      contact_info 
----+----------+------------+----+-----------+-------------------+--------------
        1 | Fulano   | 1965-06-07 |  1 |         1 | 2023-10-10        |
      867-5309
        2 | Beltrano | 1980-10-07 |  2 |         2 | 2023-10-12        |
      555-1212
      (2 rows)
      
      foo=# INSERT INTO customer (person_id, registration_date,
      contact_info)
      values (8, '2023-09-11', '(212)555-1212');
      ERROR:  insert or update on table "customer" violates foreign key
      constraint "customer_person_id_fkey"
      DETAIL:  Key (person_id)=(8) is not present in table "person".

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