Problem with group by on interval

gistfile1.sql

## Output Stock transactions


id	source_id	inventory_item_id	quantity	comment	author_id	created_at	updated_at	source_type	cause
549133	189953	149193	1		55291	"2010-12-16 12:59:23"	"2010-12-16 12:59:23"	PurchaseItem	PURCHASE
586203	203443	149193	11		55291	"2010-12-24 10:15:30"	"2010-12-24 10:15:30"	PurchaseItem	PURCHASE
586413	195123	149193	10		55291	"2010-12-24 10:15:31"	"2010-12-24 10:15:31"	PurchaseItem	PURCHASE
669023	1159453	149193	-1		2943951	"2011-01-13 10:41:42"	"2011-01-13 10:41:42"	LineItem	SALE
765663	1286113	149193	-1		1951411	"2011-02-02 15:21:14"	"2011-02-02 15:21:14"	LineItem	SALE
850583	1388293	149193	-1		4869271	"2011-03-07 08:08:11"	"2011-03-07 08:08:11"	LineItem	SALE

## QUERY TO GET DOUBLES 

select st.author_id, st.inventory_item_id, st.created_at, count(*) as total 
FROM stock_transactions st 
WHERE st.cause = 'PURCHASE'
group by st.author_id, st.inventory_item_id, st.created_at
HAVING total > 1;