Setting Variable - (Correct)

[Ranieri Mazili <ranieri.oliveira@xxxxxxxxxxxx>]

Hello, I'm trying do the following function:

CREATE OR REPLACE FUNCTION lost_hours_temp(date)
RETURNS text AS
$BODY$
DECLARE
   START_DATE date;
   END_DATE date;
   QUERY text;
BEGIN
   START_DATE := $1;
   END_DATE := START_DATE - interval '3 year';
  
   WHILE EXTRACT(YEAR FROM START_DATE) = EXTRACT(YEAR FROM END_DATE)+3 LOOP
       QUERY := 'SELECT    CAST(EXTRACT(YEAR FROM A.production_date) || 
'/' || EXTRACT(MONTH FROM A.production_date) AS TEXT) as date,
               SUM(production_hours) AS production_hours,
               B.id_production_area
           FROM     production A, product B
           WHERE     EXTRACT(MONTH FROM production_date) = 
EXTRACT(MONTH FROM ' || START_DATE || ')
               AND EXTRACT(YEAR FROM A.production_date) = EXTRACT(YEAR 
FROM ' || START_DATE || ')
               AND lost_hours = ' || 'S' ||'
               AND A.id_product = B.id_product
           GROUP BY id_production_area, date';
  
       START_DATE := START_DATE - interval '1 month';
   END LOOP;

   RETURN QUERY;
END;
$BODY$
LANGUAGE 'plpgsql';

My problem is into WHILE, I'm trying to concatenate variables with the 
string, but I guess that it's generating an error.
What's the correct form to concatenate strings with query in my case?

Thanks