Usage of sequence functions in PostgreSQL

data.sql

DROP OWNED BY optical;

CREATE TABLE producttype (id int,
  active boolean,
  CONSTRAINT pk_producttype_id PRIMARY KEY (id));

CREATE TABLE lang (id int,
  name varchar NOT NULL,
  CONSTRAINT pk_lang_id PRIMARY KEY (id),
  CONSTRAINT un_lang_name UNIQUE (name));

CREATE TABLE producttypei18n (id int,
  label varchar,
  lang varchar,-- CHECK (lang IN ('FRENCH', 'ENGLISH')),
  producttype_id int,
  CONSTRAINT pk_producttypei18n_id PRIMARY KEY (id),
  CONSTRAINT fk_producttype_id FOREIGN KEY (producttype_id)
    REFERENCES producttype (id),
  CONSTRAINT fk_lang_name FOREIGN KEY (lang)
    REFERENCES lang (name));

CREATE SEQUENCE seq_producttype_id MINVALUE 0 MAXVALUE 1023 INCREMENT BY 1;

CREATE SEQUENCE seq_lang_name MINVALUE 0 MAXVALUE 1023 INCREMENT BY 1;

CREATE SEQUENCE seq_producttypei18n_id MINVALUE 0 MAXVALUE 1023 INCREMENT BY 1;

INSERT INTO lang (id, name) VALUES (nextval('seq_lang_name'), 'FRENCH'),
  (nextval('seq_lang_name'), 'ENGLISH');

INSERT INTO producttype (id, active) VALUES
  (nextval('seq_producttype_id'), false);

INSERT INTO producttypei18n (id, label, lang, producttype_id) VALUES
  (nextval('seq_producttypei18n_id'), 'Banane', 'FRENCH', currval('seq_producttype_id')),
  (nextval('seq_producttypei18n_id'), 'Banana', 'ENGLISH', currval('seq_producttype_id'));

INSERT INTO producttype (id, active) VALUES
  (nextval('seq_producttype_id'), true);

INSERT INTO producttypei18n (id, label, lang, producttype_id) VALUES
  (nextval('seq_producttypei18n_id'), 'Pomme', 'FRENCH', currval('seq_producttype_id')),
  (nextval('seq_producttypei18n_id'), 'Apple', 'ENGLISH', currval('seq_producttype_id'));
  
SELECT * FROM producttype;
  
SELECT * FROM producttypei18n;

SELECT i.id AS pti_id, i.label, i.lang, p.id AS pt_id, p.active FROM producttypei18n i,
  producttype p;