How to create an "on-the-fly" mapping table within a SELECT statement in Postgresql

Question

I'm creating a select statement that combines two tables, zone and output, based on a referenced device table and on a mapping of zone_number to output_type_id. The mapping of zone_number to output_type_id doesn't appear anywhere in the database, and I would like to create it "on-the-fly" within the select statement. Below is my schema:

CREATE TABLE output_type (
    id INTEGER NOT NULL, 
    name TEXT,
    PRIMARY KEY (id)
);

CREATE TABLE device (
    id INTEGER NOT NULL,
    name TEXT,
    PRIMARY KEY (id)
);

CREATE TABLE zone (
    id SERIAL NOT NULL,
    device_id INTEGER NOT NULL REFERENCES device(id),
    zone_number INTEGER NOT NULL,
    PRIMARY KEY (id), 
    UNIQUE (zone_number)
);

CREATE TABLE output (
    id SERIAL NOT NULL,
    device_id INTEGER NOT NULL REFERENCES device(id),
    output_type_id INTEGER NOT NULL REFERENCES output_type(id),
    enabled BOOLEAN NOT NULL,
    PRIMARY KEY (id)
);

And here is some example data:

INSERT INTO output_type (id, name) VALUES 
(101, 'Output 1'),
(202, 'Output 2'),
(303, 'Output 3'),
(404, 'Output 4');

INSERT INTO device (id, name) VALUES 
(1, 'Test Device');

INSERT INTO zone (device_id, zone_number) VALUES 
(1, 1),
(1, 2),
(1, 3),
(1, 4);

INSERT INTO output (device_id, output_type_id, enabled) VALUES 
(1, 101, TRUE),
(1, 202, FALSE),
(1, 303, FALSE), 
(1, 404, TRUE);

I need to get the associated enabled field from the output table for each zone for a given device. Each zone_number maps to an output_type_id. For this example:

zone_number | output_type_id
----------------------------
1           | 101
2           | 202
3           | 303 
4           | 404

One way to handle the mapping would be to create a new table

CREATE TABLE zone_output_type_map (
    zone_number INTEGER,
    output_type_id INTEGER NOT NULL REFERENCES output_type(id)
);

INSERT INTO zone_output_type_map (zone_number, output_type_id) VALUES 
(1, 101),
(2, 202),
(3, 303), 
(4, 404);

And use the following SQL to get all zones, plus the enabled flag, for device 1:

SELECT zone.*, output.enabled 
FROM zone
JOIN output 
ON output.device_id = zone.device_id
JOIN zone_output_type_map map
ON map.zone_number = zone.zone_number
AND map.output_type_id = output.output_type_id
AND zone.device_id = 1

However, I'm looking for a way to create the mapping of zone nunbers to output types without creating a new table and without piecing together a bunch of AND/OR statements. Is there an elegant way to create a mapping between the two fields within the select statement? Something like:

SELECT zone.*, output.enabled 
FROM zone
JOIN output 
ON output.device_id = zone.device_id
JOIN (
    SELECT (
        1 => 101,
        2 => 202,
        3 => 303,
        4 => 404
    ) (zone_number, output_type_id)
) as map
ON map.zone_number = zone.zone_number
AND map.output_type_id = output.output_type_id
AND zone.device_id = 1

Disclaimer: I know that ideally the enabled field would exist in the zone table. However, I don't have control over that piece. I'm just looking for the most elegant solution from the application side. Thanks!

Answer 1

You can use VALUES as an inline table and JOIN to it, you just need to give it an alias and column names:

join (values (1, 101), (2, 202), (3, 303), (4, 304)) as map(zone_number, output_type_id)
on ...

From the fine manual:

VALUES can also be used where a sub-SELECT might be written, for example in a FROM clause:

SELECT f.*
  FROM films f, (VALUES('MGM', 'Horror'), ('UA', 'Sci-Fi')) AS t (studio, kind)
  WHERE f.studio = t.studio AND f.kind = t.kind;

UPDATE employees SET salary = salary * v.increase
  FROM (VALUES(1, 200000, 1.2), (2, 400000, 1.4)) AS v (depno, target, increase)
  WHERE employees.depno = v.depno AND employees.sales >= v.target;