<column> is ambiguous in column comparison between two tables - sql

Question

I want this postgres function to work :
CREATE OR REPLACE FUNCTION difference_of_match_ids_in_match_history_and_match_results()
returns table(match_id BIGINT)
as
$$
BEGIN
return QUERY
SELECT *
FROM sports.match_history
WHERE match_id NOT IN (SELECT match_id
FROM sports.match_results);
END $$
LANGUAGE 'plpgsql';
This stand alone query works just fine:
SELECT *
FROM sports.match_history
WHERE match_id NOT IN (SELECT match_id FROM sports.match_results);
But when I put it into this function and try to run it like this:
select *
from difference_of_match_ids_in_match_history_and_match_results();
I get this:
SQL Error [42702]: ERROR: column reference "match_id" is ambiguous
Detail: It could refer to either a PL/pgSQL variable or a table
column. Where: PL/pgSQL function
difference_of_match_ids_in_match_history_and_match_results() line 3 at
RETURN QUERY
I've seen other questions with this same error, and they suggest naming the sub queries to specify which instance of a column you're referring to, however, those examples use joins and my query works fine outside of the function.
If I do need to name the column, how would I do so with only one sub-query?
If that isn't the issue, then I'm assuming that there's something wrong with the way I'm defining a function.

Answer 1

You query is fine. The ambiguity is on the match_id in returns table(match_id BIGINT) rename it or prefix the columns with the table name in your query
CREATE OR REPLACE FUNCTION difference_of_match_ids_in_match_history_and_match_results()
returns table(new_name BIGINT)
as
$$
BEGIN
return QUERY
SELECT *
FROM sports.match_history
WHERE match_id NOT IN (SELECT match_id
FROM sports.match_results);
END $$
LANGUAGE 'plpgsql';
or
CREATE OR REPLACE FUNCTION difference_of_match_ids_in_match_history_and_match_results()
returns table(match_id BIGINT)
as
$$
BEGIN
return QUERY
SELECT sports.match_history.match_id
FROM sports.match_history
WHERE sports.match_history.match_id NOT IN (SELECT sports.match_results.match_id
FROM sports.match_results);
END $$
LANGUAGE 'plpgsql';
Didn't test the code.

Answer 2

The structure of the result set must match the function result type. If you want to get only match_ids:
CREATE OR REPLACE FUNCTION difference_of_match_ids_in_match_history_and_match_results()
RETURNS TABLE(m_id BIGINT) -- !!
AS
$$
BEGIN
RETURN QUERY
SELECT match_id -- !!
FROM sports.match_history
WHERE match_id NOT IN (SELECT match_id
FROM sports.match_results);
END $$
LANGUAGE 'plpgsql';
If you want to get whole rows as a result:
DROP FUNCTION difference_of_match_ids_in_match_history_and_match_results();
CREATE OR REPLACE FUNCTION difference_of_match_ids_in_match_history_and_match_results()
RETURNS SETOF sports.match_history -- !!
AS
$$
BEGIN
RETURN QUERY
SELECT * -- !!
FROM sports.match_history
WHERE match_id NOT IN (SELECT match_id
FROM sports.match_results);
END $$
LANGUAGE 'plpgsql';

Answer 3

As others have answerd, it's an ambiguity between the result definition and PL/pgSQL variables. The column name in a set returning function is in fact also a variable inside the function.
But you don't need PL/pgSQL for this in the first place. If you use a plain SQL function it will be more efficient and the problem will go away as well:
CREATE OR REPLACE FUNCTION difference_of_match_ids_in_match_history_and_match_results()
returns table(match_id BIGINT)
as
$$
SELECT match_id --<< do not return * - only return one column
FROM sports.match_history
WHERE match_id NOT IN (SELECT match_id
FROM sports.match_results);
$$
LANGUAGE sql;
Note that the language name is an identifier and should not be quoted at all.

Answer 4

The naming conflict between column names and plpgsql OUT parameters has been addressed. More details here:
Postgresql - INSERT RETURNING INTO ambiguous column reference
I would also use a different query style. NOT IN (SELECT ...) is typically slowest and carries traps with NULL values. Use NOT EXISTS instead:
SELECT match_id
FROM sports.match_history h
WHERE NOT EXISTS (
SELECT match_id
FROM sports.match_results
WHERE match_id = h.match_id
);
More:
Select rows which are not present in other table