Concatenate variable into dynamic SELECT statement in PL/pgSQL - sql

Question

I want to use a variable in a dynamic SELECT statement (that is in an EXECUTE statement) like this:
CREATE OR REPLACE FUNCTION get_factor()
RETURNS TABLE(factor numeric) AS
$BODY$
DECLARE
_query character varying;
_some_condition integer := 50;
_result decimal;
BEGIN
_query := 'SELECT factors.factor_material FROM factors
WHERE factors.condition = _some_condition;';
EXECUTE _query INTO _result;
RETURN QUERY SELECT _result;
END;
I have a variable _some_condition integer := 50; and I want to concatenate its value into the SELECT statement (...WHERE factors.condition = _some_condition;) however this is giving me a "column does no exist" error:
ERROR: column "_some_condition" does not exist
LINE 1: ...erial WHERE factors.city_id = _some_cond...
^
CONTEXT: PL/pgSQL function get_factor() line 12 at EXECUTE statement
Why am I getting this error and how to fix this? Keep in mind I have to use a dynamic SELECT statement.

Answer 1

Since you are trying to pass a value (which is not the same as a literal, btw), all you need is the USING clause of the plpgsql EXECUTE command, like #Craig commented.
While being at it, simplify your function with RETURN QUERY EXECUTE:
CREATE OR REPLACE FUNCTION get_factor()
RETURNS TABLE(factor numeric) AS
$func$
DECLARE
_some_condition integer := 50;
BEGIN
RETURN QUERY EXECUTE
'SELECT factor_material
FROM factors
WHERE condition = $1'
USING _some_condition;
END
$func$ LANGUAGE plpgsql;
Nothing in this example requires dynamic SQL. You could just:
RETURN QUERY
SELECT f.factor_material
FROM factors f
WHERE f.condition = _some_condition;