Check if row already exists, if so tell the referenced table the id

Question

Let's assume I have a table magazine:

CREATE TABLE magazine
(
  magazine_id integer NOT NULL DEFAULT nextval(('public.magazine_magazine_id_seq'::text)::regclass),
  longname character varying(1000),
  shortname character varying(200),
  issn character varying(9),
  CONSTRAINT pk_magazine PRIMARY KEY (magazine_id)
);

And another table issue:

CREATE TABLE issue
(
  issue_id integer NOT NULL DEFAULT nextval(('public.issue_issue_id_seq'::text)::regclass),
  number integer,
  year integer,
  volume integer,
  fk_magazine_id integer,
  CONSTRAINT pk_issue PRIMARY KEY (issue_id),
  CONSTRAINT fk_magazine_id FOREIGN KEY (fk_magazine_id)
      REFERENCES magazine (magazine_id) MATCH SIMPLE
      ON UPDATE NO ACTION ON DELETE NO ACTION
);

Current INSERTS:

INSERT INTO magazine (longname,shotname,issn)
VALUES ('a long name','ee','1111-2222');

INSERT INTO issue (fk_magazine_id,number,year,volume)
VALUES (currval('magazine_magazine_id_seq'),'8','1982','6');

Now a row should only be inserted into 'magazine', if it does not already exist. However if it exists, the table 'issue' needs to get the 'magazine_id' of the row that already exists in order to establish the reference.

How can i do this?

Thx in advance!

Answer 1

I'm not sure that if you can do this with SQL. I know that Oracle can be used for triggers, but i don't think SQL is able to. Someone correct me if I'm wrong.

Answer 2

How do you know if a magazine is already in magazine table? Does issn column define a magazine? If yes then it should be a primary key, or at least unique.

The easiest way would be to do a check for magazine existence in your client application, like this (in pseudocode):

function insert_issue(longname, shotname, issn, number,year,volume) {
    /* extensive comments for newbies */
    start_transaction();
    q_get_magazine_id = prepare_query(
      'select magazine_id from magazine where issn=?'
    );
    magazine_id = execute_query(q_get_magazine_id, issn);
    /* if magazine_id is null now then there’s no magazine with this issn */
    /* and we have to add it */
    if ( magazine_id == NULL ) {
      q_insert_magazine = prepare_query(
        'insert into magazine (longname, shotname, issn)
          values (?,?,?) returning magazine_id'
      );
      magazine_id = execute_query(q_insert_magazine, longname, shortname, issn);
      /* we have tried to add a new magazine; */
      /* if we failed (magazine_id==NULL) then somebody else just added it */
      if ( magazine_id == NULL ) { 
        /* other, parerelly connected client just inserted this magazine, */
        /* this is unlikely but possible */
        rollback();
        start_transaction();
        magazine_id = execute_query(q_get_magazine_id, issn);
      }
    }
    /* now magazine_id is an id of magazine, */
    /* added if it was not in a database before, new otherwise */
    q_insert_issue = prepare_query(
      'insert into issue (fk_magazine_id,number,year,volume)
         values (?,?,?,?)'
    );
    execute_query(q_insert_issue, magazine_id, number, year, volume);
    /* we have inserted a new issue referencing old, */
    /* or if it was needed new, magazine */
    if ( ! commit() ) {
      rollback();
      raise "Unable to insert an issue";
    }
}

If you just have to do this in one query then you can implement this pseudocode as pl/pgsql function in database and just select insert_issue(?, ?, ?, ?, ?, ?).