Started                 UserId
"2012-09-25 00:01:04"   164382
"2012-09-25 00:01:39"   164382
"2012-09-25 00:02:37"   166121
"2012-09-25 00:03:35"   155682
"2012-09-25 00:04:18"   160947
"2012-09-25 00:08:19"   165806

SELECT COUNT(DISTINCT UserId)
FROM Session
WHERE Started BETWEEN '2012-09-18 00:00' AND '2012-09-25 00:00';

SELECT
DATE(A.Started),
Count(DISTINCT A.UserId)
FROM Session AS A
WHERE DATE(A.Started) BETWEEN DATE(DATE_SUB(DATE(DATE(A.Started)),INTERVAL 7 DAY)) AND DATE(DATE(A.Started))
GROUP BY DATE(A.Started)
ORDER BY DATE(A.Started);

DATE(A.Started) "Count(DISTINCT A.UserId)"
2012-09-18          709
2012-09-19          677
2012-09-20          658
2012-09-21          556
2012-09-22          530
2012-09-23          479
2012-09-24          528
2012-09-25          480
...

SELECT
    DATE_FORMAT(d1.Started, '%Y-%m-%d') AS Started,
    COUNT(DISTINCT d2.UserID) Users
FROM
(
    SELECT
        DATE(Started) AS Started
    FROM
        Session
    GROUP BY
        DATE(Started)
) d1
INNER JOIN
(
    SELECT DISTINCT
        DATE(Started) AS Started,
        UserID
    FROM
        Session
) d2
ON d2.Started BETWEEN d1.Started - INTERVAL 7 DAY AND d1.Started
GROUP BY
    d1.Started
ORDER BY
    d1.Started DESC

Select Distinct Date(A.Started), Count(B.UserId)
From Session a
    Join Session b
        On b.Start Between AddDate(A.Start, day, -7) And A.Start