使用人数统计的SQL - MS-SQL Server / 疑难问题

SQL code:

create table #tb1(ip nvarchar(20),date datetime)
insert into #tb1
select '10.110.120.114','2010-04-01 03:57:49' union all
select '10.119.123.130','2010-04-01 04:01:22' union all
select '10.110.120.114','2010-04-01 04:04:51' union all
select '10.110.120.114','2010-04-01 04:11:42' union all
select '10.128.130.110','2010-04-01 04:04:51' union all
select '10.110.120.114','2010-04-01 06:48:42' union all
select '10.110.120.114','2010-04-01 06:57:45' union all



求实际使用人数.
以第一次出现的IP为准（group by IP）,半小时内的都当做他一直在使用,半小时后出现的才算第二次使用。
以10.110.120.114举例,第一次是 03:57:49,此后半小时内出现的 04:04:51\04:11:42就不做统计了.(03:57:49)的半小时后最近的一个也就是 06:48:42,用它再做起点,06:57:45就又不统计……以此类推
求个IP实用的次数
结果应该是这样的

ip counts
-------------- -----------
10.110.120.114 2
10.119.123.130 1
10.128.130.110 1

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SQL code:
create table #tb1(ip nvarchar(20),date datetime)
insert into #tb1
select '10.110.120.114','2010-04-01 03:57:49' union all
select '10.119.123.130','2010-04-01 04:01:22' union all
select '10.110.120.114','2010-04-01 04:04:51' union all
select '10.110.120.114','2010-04-01 04:11:42