Sequences

CREATE TABLE visitor(
      id SERIAL PRIMARY KEY
    , some_unique_textual_property VARCHAR (42) UNIQUE
);
 
-- single sequence will be used to autoincrement both single_ticket and multiple_ticket tables
CREATE SEQUENCE unique_id_seq;
 
CREATE TABLE single_ticket(
      id INT4 PRIMARY KEY DEFAULT NEXTVAL('unique_id_seq')
    , visitor_id INT4 REFERENCES visitor(id)
);
 
-- just to not to forget to drop it, when the time will come)
ALTER SEQUENCE unique_id_seq OWNED BY single_ticket.id;
 
CREATE TABLE multiple_ticket(
      id INT4 PRIMARY KEY DEFAULT NEXTVAL('unique_id_seq')
    , visitor_id INT4 REFERENCES visitor(id)
);