CREATE OR REPLACE FUNCTION PlusGrosClient() RETURNS trigger AS

       'BEGIN
            SELECT t.IDClient FROM Composition t NATURAL JOIN Produit u
                GROUP BY IDClient
                    WHERE NOT EXISTS (
                        SELECT v.IDClient FROM Composition v NATURAL JOIN Produit w WHERE

                            SELECT * FROM Commande x
                                WHERE v.IDClient = x.IDClient
                                SUM x.Quantite*Prix> t.Quantite*Prix
                                )

            UPDATE
        END;'

    LANGUAGE 'plpgsql';