Is there pattern to have union table for different items?

Question

I\'d like to have column constraint based combination of 2 columns. I don\'t find the way to use foreign key here, because it should be conditional FK, then. Hope this basic SQL

Answer 1

Based on @Laurenz Albe answer I form a solution for example above. Main difference: there is parent table performer, which PK is FK/PK for specific performer-tables and is referenced also from gig table.

CREATE TABLE performer_type (
  id serial primary key,
  type varchar
);
INSERT INTO performer_type ( id, type ) VALUES (1, 'singer' ), ( 2, 'band' );

CREATE TABLE performer (
  id serial primary key,
  performer_type_id int REFERENCES performer_type(id)
);

CREATE TABLE singer (
  id int primary key REFERENCES performer(id),
  name varchar
);

INSERT INTO performer ( performer_type_id ) VALUES (1); -- get PK 1 for next statement
INSERT INTO singer ( id, name ) VALUES (1, 'Robert');

CREATE TABLE band (
  id int primary key REFERENCES performer(id),
  name varchar
);

INSERT INTO performer ( performer_type_id ) VALUES (2); -- get PK 2 for next statement
INSERT INTO singer ( id, name ) VALUES (2, 'Animates');
INSERT INTO performer ( performer_type_id ) VALUES (2); -- get PK 3 for next statement
INSERT INTO singer ( id, name ) VALUES (3, 'Zed Leppelin');

CREATE TABLE gig (
  id serial primary key,
  performer_id int REFERENCES performer(id)
);

INSERT INTO gig ( performer_id ) VALUES (1), (2), (3), (4);

And the last INSERT fails, as expected:

ERROR:  insert or update on table "gig" violates foreign key constraint "gig_performer_id_fkey"
DETAIL:  Key (performer_id)=(4) is not present in table "performer".

But

For me there is annoying problem: I have no good way to make distinction which ID is for singer and which for band etc. (in original example I had performer_type_id in gig-table for that), because any performer_id may belong any performer. So I'd like any performer type has it's own ID range, so I create dummy table for every sequence

CREATE TABLE band_id (
  id int primary key,
  dummy boolean default null
);
CREATE SEQUENCE band_id_seq START 1;
ALTER TABLE band_id ALTER COLUMN id SET DEFAULT nextval('band_id_seq');

CREATE TABLE singer_id (
  id int primary key,
  dummy boolean default null
);
CREATE SEQUENCE singer_id_seq START 2000000;
ALTER TABLE singer_id ALTER COLUMN id SET DEFAULT nextval('singer_id_seq');

Now, to insert new row into specific perfomer table I have to get next ID for it:

INSERT INTO band_id (dummy) VALUES (NULL);

Trying to figure out, is it possible to solve this process on DB level, or has something to done in App-level. It would be nice, if inserting into band table could:

• before trigger inserting into band_id to genereate specific ID
• before trigger inserting this new ID into performer-table
• include this new ID into INSERT into band

Frist 2 points are easy, but the last point is not clear for now.