SQL Server how to select first row in sequence

Question

I have a table with the following sample data:

Tag          Loc        Time1
A            10         6/2/15 8:00 AM
A            10         6/2/15 7:50 AM
A            10         6/2/15 7:30 AM
A            20         6/2/15 7:20 AM
A            20         6/2/15 7:15 AM
B            10         6/2/15 7:12 AM
B            10         6/2/15 7:11 AM
A            10         6/2/15 7:10 AM
A            10         6/2/15 7:00 AM

I need SQL to select the first (earliest) row in a sequence until location changes, then select the earliest row again until location changes. In other words I need the following output from above:

Tag         Loc         Time1
A           10          6/2/15 7:30 AM
A           20          6/2/15 7:15 AM
A           10          6/2/15 7:00 AM
B           10          6/2/15 7:11 AM

I tried this from Giorgos - but some lines from the select were duplicated:

declare @temptbl table (rowid int primary key identity, tag nvarchar(1), loc int, time1 datetime)
declare @tag as nvarchar(1), @loc as int, @time1 as datetime

insert into @temptbl (tag, loc, time1) values (1,20,'6/5/2015 7:15 AM')
insert into @temptbl (tag, loc, time1) values (1,20,'6/5/2015 7:20 AM')
insert into @temptbl (tag, loc, time1) values (1,20,'6/5/2015 7:25 AM')
insert into @temptbl (tag, loc, time1) values (4,20,'6/5/2015 7:20 AM')
insert into @temptbl (tag, loc, time1) values (4,20,'6/5/2015 7:25 AM')
insert into @temptbl (tag, loc, time1) values (4,20,'6/5/2015 7:30 AM')
insert into @temptbl (tag, loc, time1) values (4,20,'6/5/2015 7:35 AM')
insert into @temptbl (tag, loc, time1) values (4,20,'6/5/2015 7:40 AM')

select * from @temptbl

SELECT Tag, Loc, MIN(Time1) as time2
FROM (
SELECT Tag, Loc, Time1,      
       ROW_NUMBER() OVER (ORDER BY Time1) - 
       ROW_NUMBER() OVER (PARTITION BY Tag, Loc 
                          ORDER BY Time1) AS grp
FROM @temptbl ) t

GROUP BY Tag, Loc, grp

Here is the results (there should only be one line for each tag)

Tag  Loc time2
1   20  2015-06-05 07:15:00.000
1   20  2015-06-05 07:25:00.000
4   20  2015-06-05 07:20:00.000
4   20  2015-06-05 07:30:00.000

Answer 1

Assuming you're using MS SQL Server 2012 or newer, the lag window function will allow you to compare a row to the previous one:

SELECT tag, loc, time1
FROM   (SELECT tag, loc, time1,
               LAG (loc) OVER (PARTITION BY tag ORDER BY time1) AS lagloc
        FROM   my_table) t 
WHERE  loc != lagloc OR lagloc IS NULL