What you need to do is round c.time
down to the nearest 30 minutes interval, and round c.until
up to the nearest interval. This way your where
clause will have the correct range.
To do the rounding you will need to convert the times to datetime
which you can do like so:
CAST(CONVERT(varchar,THE_TIME_AS_VARCHAR,121) AS datetime)
Then you can round down to the nearest 30 minutes like so:
DATEADD(mi, DATEDIFF(mi, 0, THE_TIME_AS_DATETIME)/30*30, 0)
And round up like so:
DATEADD(mi, DATEDIFF(mi, 30, THE_TIME_AS_DATETIME)/30*30, 0)
Applying all that to your existing code would give you this:
select t.time,
isnull(
(select c.studentCode
from tbclassrsv c
where c.transdate='2014-02-05 00:00:00'
and c.class='ROOM 01'
and c.status<>'DEL'
and t.time>= DATEADD(mi, DATEDIFF(mi, 0, CAST(CONVERT(varchar,c.time,121) AS datetime))/30*30, 0)
and t.time<= DATEADD(mi, DATEDIFF(mi, 30, CAST(CONVERT(varchar,c.until,121) AS datetime))/30*30, 0)
),'-') [Student]