Looking at grouping circ counts by "week" with some consideration given to avoiding partial weeks at the start of the data summarized.

circs_by_week.sql

-- This will show counts grouped by weeks, but will
-- likely include a partial week at the start
-- (check the "days_in_sample" column)
SELECT date_trunc('week',xact_start)::date week,
circ_lib,
count(distinct xact_start::date) days_in_sample,
count(*) circ_count
FROM action.circulation
WHERE xact_start > '2012-12-01'
GROUP BY 1,2
ORDER BY 1,2;

-- This will ensure that we have a 
-- full week at the start of the data
-- (assuming that is possible)
-- again, see the "days_in_sample" column
SELECT date_trunc('week',xact_start)::date week,
circ_lib,
count(distinct xact_start::date) days_in_sample,
count(*) circ_count
FROM action.circulation
WHERE xact_start > '2012-12-01'
GROUP BY 1,2
HAVING date_trunc('week',xact_start)::date > '2012-12-01'
ORDER BY 1,2;


-- The example just before this excludes partial
-- weeks, at the expense of excluding data within
-- the range we requested.
-- This example favors pulling earlier than the
-- specified date in order to ensure a full week
SELECT date_trunc('week',xact_start)::date week,
circ_lib,
count(distinct xact_start::date) days_in_sample,
count(*) circ_count
FROM action.circulation
WHERE xact_start::date >= date_trunc('week', '2012-12-01'::date)::date
GROUP BY 1,2
ORDER BY 1,2;