I've got this table.

CREATE TABLE VARIACION (
id_art "ID" not null,
id_suc "ID" not null,
variacion numeric(4,2)not null,
fecha date,
PRIMARY KEY (id_art,id_suc));

How could y query this table grouping by
for examples by weeks.
for example
the VARIACION of some id_art betwen fecha X and Fecha X
but this result i must group by week.

Excuse my english, Im from Argentina
Thanks.