Edit 1 (clarification): Thank you for the answers so far! The response is gratifying. I want to clarify the question a little because based on the answers I think I did not describe one aspect of the problem correctly (and I'm sure that's my fault as I was having a difficult time defining it even for myself). Here's the rub: The result set should contain ONLY the records with tstamp BETWEEN '2010-01-03' AND '2010-01-09', AND the one record where the tstamp IS NULL for each order_num in the first set (there will always be one with null tstamp for each order_num). The answers given so far appear to include all records for a certain order_num if there are any with tstamp BETWEEN '2010-01-03' AND '2010-01-09'. For example, if there were another record with order_num = 2 and tstamp = 2010-01-12 00:00:00 it should not be included in the result. Original question: Consider an orders table containing id (unique), order_num, tstamp (a timestamp), and item_id (the single item included in an order). tstamp is null, unless the order has been modified, in which case there is another record with identical order_num and tstamp then contains the timestamp of when the change occurred. Example... id order_num tstamp item_id __ _________ ___________________ _______ 0 1 100 1 2 101 2 2 2010-01-05 12:34:56 102 3 3 113 4 4 124 5 5 135 6 5 2010-01-07 01:23:45 136 7 5 2010-01-07 02:46:00 137 8 6 100 9 6 2010-01-13 08:33:55 105 What is the most efficient SQL statement to retrieve all of the orders (based on order_num) which have been modified one or more times during a certain date range? In other words, for each order we need all of the records with the same order_num (including the one with NULL tstamp), for each order_num WHERE at least one of the order_num's has tstamp NOT NULL AND tstamp BETWEEN '2010-01-03' AND '2010-01-09'. It's the "WHERE at least one of the order_num's has tstamp NOT NULL" that I'm having difficulty with. The result set should look like this: id order_num tstamp item_id __ _________ ___________________ _______ 1 2 101 2 2 2010-01-05 12:34:56 102 5 5 135 6 5 2010-01-07 01:23:45 136 7 5 2010-01-07 02:46:00 137 The SQL that I came up with is this, which is essentially "A UNION (B in A)", but it executes slowly and I hope there is a more efficient solution: SELECT history_orders.order_id, history_orders.tstamp, history_orders.item_id FROM (SELECT orders.order_id, orders.tstamp, orders.item_id FROM orders WHERE orders.tstamp BETWEEN '2010-01-03' AND '2010-01-09') AS history_orders UNION SELECT current_orders.order_id, current_orders.tstamp, current_orders.item_id FROM (SELECT orders.order_id, orders.tstamp, orders.item_id FROM orders WHERE orders.tstamp IS NULL) AS current_orders WHERE current_orders.order_id IN (SELECT orders.order_id FROM orders WHERE orders.tstamp BETWEEN '2010-01-03' AND '2010-01-09');