How to join on to variable table based on field value? - sql

Question

I have a table called tracks with with basic data. The important parts are that this table has a column named id and event. The value of the event field is the name of another table. That table has a matching id with a lot of details about what was tracked. Is it possible to do something like this?
SELECT id, event, e.*
FROM tracks t
LEFT JOIN $event e ON t.id = e.id
The value of event could be one of a hundred different values.

Answer 1

Probably you could use inheritance: PostgreSQL 9.5.1 Documentation: Inheritance
In this case you may have empty parent table:
CREATE TABLE events(
id SERIAL PRIMARY KEY,
event_type INTEGER,
...
);
And bunch of children tables
CREATE TABLE events_1(event_type INTEGER DEFAULT 1 CHECK(event_type = 1)) INHERITS (events);
CREATE TABLE events_2(event_type INTEGER DEFAULT 2 CHECK(event_type = 2)) INHERITS (events);
...
Then you will be able to use queries like:
SELECT t.id, t.event_type, e.*
FROM tracks t
JOIN events e on t.id = e.id AND t.event_type = e.event_type;
But you have to add all columns to parent if you want request them from parent table.
UPD: you cannot use variable as table name in pure sql. Only way to do it is dynamic code generation (for example in plpgsql).

Answer 2

You can't have a variable table name in a simple SQL query; one way or another, you need to build and execute a dynamic query string.
If you want to avoid doing this in your application code, you'll need to use PL/pgSQL's EXECUTE, which means you'll have to create a function to do it.
If the common fields in your event tables are e.g. x INT, y INT, then this should work (though it may not be particularly efficient):
CREATE FUNCTION EventTable(TableName TEXT, id INT) RETURNS TABLE (x INT, y INT) AS
$$
BEGIN
RETURN QUERY EXECUTE 'SELECT x, y FROM ' || TableName || ' WHERE id = $1' USING id;
END
$$
LANGUAGE plpgsql;
SELECT id, event, e.*
FROM t
LEFT JOIN EventTable(event, id) e ON true;
SQLFiddle example