|       DATE          |  USER_ID  | GROUP_ID   |
| 2012-09-20 00:00:00 |     7     |     1      |
| 2012-09-20 00:00:00 |     6     |     1      |
| 2012-09-13 00:00:00 |     5     |     1      |
| 2012-09-10 00:00:00 |     1     |     1      |
| 2012-09-02 00:00:00 |     5     |     3      |
| 2012-08-02 00:00:00 |     5     |     2      |
| 2012-07-01 00:00:00 |     5     |     1      |
| 2012-07-01 00:00:00 |     3     |     1      |

SELECT *
FROM user_group
where group_id = 1
and '2012-09-11' <= date
and date <= '2012-09-20'
UNION
SELECT TOP 1 *
FROM user_group
where group_id = 1
and date <= '2012-09-11'
order by date desc

|       DATE          |  USER_ID  | GROUP_ID   |
| 2012-09-20 00:00:00 |     7     |     1      |
| 2012-09-20 00:00:00 |     6     |     1      |
| 2012-09-13 00:00:00 |     5     |     1      |
| 2012-09-10 00:00:00 |     1     |     1      |
| 2012-07-01 00:00:00 |     3     |     1      |

select * from user_group where group_id=1 and date<=convert(date,'2012-09-20',120)
except
select * from user_group where group_id=1 and date<=convert(date,'2012-09-11',120)

select date, user_id, group_id
from
(
    select *, ROW_NUMBER() over (partition by user_id order by date desc) rn
    from user_group
    where date<=convert(date,'2012-09-20',120)
) v
where rn=1
and group_id=1
order by date desc

db.user_group.Where(g => g.group_id = group_id &&
                         g.date =< to_date);

var innerAssoc = from assoc in user_group
                 where assoc.group_id == group_id
                 && dateFrom.Date <= assoc.date.Date
                 && date.Date <= dateTo.Date
                 select assoc;

var outerAssoc = from assoc in user_group
                 where assoc.group_id == group_id
                 && assoc.date.Date == user_group
                 .Where(a => a.user_id == assoc.user_id && a.date.Date <= dateFrom.Date)
                 .Select(a => a.date.Date).Max()
                 select assoc;

var res = innerAssoc.Union(outerAssoc);