fix: group piece together

PostgreSQL/src/functions.sql

@@ -2,6 +2,7 @@
 CREATE OR REPLACE FUNCTION pieces_membre(int) RETURNS TABLE(id_piece int, quantite_piece int)
 AS 
 $$ 
+SELECT id_piece, SUM(quantite_pieces_possedees) FROM (
   SELECT id_piece, quantite_contenir * quantite_enregistrer as quantite_pieces_possedees
   FROM enregistrer
   NATURAL JOIN boites
@@ -9,7 +10,9 @@ NATURAL JOIN contenir
   WHERE id_membre=$1
   UNION
   (SELECT id_piece, quantite_acheter as quantite_pieces_possedees
-  FROM acheter WHERE id_membre=$1);
+    FROM acheter WHERE id_membre=$1)
+  )
+  GROUP BY id_piece
 $$
 LANGUAGE SQL;
 ;