How to Retrieve and Combine Record Fields from Multiple PostgreSQL Tables using PL/pgSQL?

Retrieving Record Fields from Multiple Tables in PostgreSQL with PL/pgSQL

Returning multiple fields from different tables as a single record in PL/pgSQL requires constructing a custom record with the desired field composition. Here's how to achieve this:

Creating a Record Data Type

Define a record type using the CREATE TYPE statement to specify the fields and their data types:

CREATE TYPE my_record_type AS (
  field1 text,
  field2 integer,
  field3 boolean
);

Implementing the Function

In the SP, use the DECLARE statement to declare a variable of the record type and populate its fields:

DECLARE
  result my_record_type;
BEGIN
  SELECT field1, field2, field3 INTO result FROM table1;
  -- Retrieve and populate additional fields from other tables
  RETURN result;
END

Handling Multi-Row Results

To retrieve multiple rows from a table and aggregate them into a single record, consider using a subquery. For example, to retrieve and flatten rows from the user table:

DECLARE
  result my_record_type;
BEGIN
  SELECT id, name FROM (
    SELECT id, name
    FROM user
    WHERE school_id = InputSchoolId
    LIMIT 2
  ) AS subquery
  INTO result;
RETURN result;
END

Using the Record Return Type

After defining the function, call it to retrieve the record:

SELECT get_object_fields('name') AS record_output;

The result will be a record containing the fields from the specified tables.

Note

While using CREATE TYPE for polymorphic results is discouraged, you can leverage the RECORD type for flexibility. For example:

CREATE FUNCTION test_ret(a text, b text) RETURNS RECORD AS $$
DECLARE 
  result RECORD;
BEGIN
  IF LENGTH(a) < LENGTH(b) THEN
    result = (TRUE, (a || b)::text, 'a shorter than b'::text);
  ELSE
    result = (FALSE, (b || a)::text, NULL::text);
  END IF;
RETURN result;
END;$$ LANGUAGE plpgsql;

This allows returning records with varying numbers of columns.

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