SELECT
    EXTRACT(week FROM SUBQ.trunc_date) AS week_number,
    AVG(SUBQ.transaction_per_day) AS avg_week_transaction
FROM
	(SELECT
		COUNT(distinct id_transaction) as transaction_per_day,
		DATE_TRUNC('day', date) AS trunc_date
	FROM
		transactions
	GROUP BY
		trunc_date) AS SUBQ

GROUP BY
    week_number