not returning 0 if value is NULL Everything works as intended, but function should return 0 if value is NULL. Currently it returns nothing (see problem at the end of the post).
How could I make to return 0 if no order is found?
'orders' Table
CREATE TABLE orders (
    id INT NOT NULL, 
    code INT NOT NULL, 
    service_id INT NOT NULL, 
    status CHARACTER VARYING(50), 
    creation TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
    status_date TIMESTAMP,
    created_user INT
    );

Records
INSERT INTO orders 
(id, code, service_id, status, creation, status_date, created_user) 
VALUES 
(100,2394,558151,'Requested','2019-06-16 11:12','2019-06-18 14:08',1),
(100,2394,558151,'Fulfilled','2018-06-16 11:12','2018-06-18 14:08',1),
(100,2394,558151,'Requested','2019-06-16 11:12','2019-06-18 14:08',1),
(100,2395,558152,'Requested','2019-06-16 11:12','2019-06-18 14:08',1),
(100,2396,558153,'Requested','2019-06-16 11:12','2019-06-18 14:08',1),
(100,2394,558151,'Requested','2019-06-16 11:12','2019-06-18 14:08',1);

Testing Statement
SELECT EXTRACT(YEAR FROM creation) AS year, 
                        orders.code, 
                        COUNT(orders.code) 
                FROM orders
                WHERE status = 'Requested' AND EXTRACT(YEAR FROM creation) = 2019 AND orders.code = 2394
                GROUP BY year, orders.code
                ORDER BY year ASC

Returns


year | code | count
:--- | ---: | ----:
2019 | 2394 |     3


Function
CREATE OR REPLACE FUNCTION 
yearly_orders(inyear numeric DEFAULT 0, incode INT DEFAULT 0) 
RETURNS TABLE (retyear numeric, retcode bigint, retcout bigint) AS 
$$
DECLARE
    retcount int DEFAULT 0;
    retcode int DEFAULT 0;
    retyear numeric DEFAULT 0;
BEGIN
    RETURN QUERY SELECT CAST(EXTRACT(year FROM creation) as numeric), 
                        CAST(orders.code as bigint), 
                        COUNT(orders.code)
                FROM orders
                WHERE status = 'Requested' 
                AND EXTRACT(YEAR FROM creation) = inyear 
                AND orders.code = incode
                GROUP BY EXTRACT(year FROM creation), orders.code
                ORDER BY retyear ASC;

END;
$$
LANGUAGE plpgsql;

Executing Function
select * from yearly_orders(2019, 1194);

Returns &lt;-- PROBLEM


retyear | retcode | retcout
------: | ------: | ------:
        |         |       


SHOULD RETURN


retyear | retcode | retcout
------: | ------: | ------:
 0       |     0    |    0  


db&lt;&gt;fiddle
The db&lt;&gt;fiddle to try this out can be found here.
The code was made by user:15356 (Here)
You are wanting GROUP BY and the COUNT aggregate to return a row with a zero value when there are otherwise no rows to return - GROUPing does not work like that. It combines what rows would have been returned if GROUP BY was not present, so no rows in that case.
You could add a second SELECT, combined with UNION ALL, that returns one row containing inyear, incode, 0 if there are zero matching rows (counted by sub-select). If there are rows this part returns nothing, if there are no matches it returns a row indicating that. Note that this may be quite inefficient.