To write a query to display records for last 15 mi

The Table(Log Analyzer) structure is :

I want to draw a line graph displaying the number of Sessions for last 15 minute to the current time (every minute). I want to write a query which displays two columns with the following information:

Date:Hour:Minute
Number of Sessions

I have tried to write a sample query displaying requests per hour:

select  convert(nvarchar(16), L.TimeLog, 120) requestTime ,(select Count(SessionID) from LogData where TimeLog <  convert(nvarchar(16), TimeLog, 120) and TimeLog >  DATEADD (mi , -15 , convert(nvarchar(16), TimeLog, 120) ) ) AS Sessions
from  LogData L
group by convert(nvarchar(16), TimeLog, 120) order by requestTime;

I tried to create a query similar to above but it shows wrong results . It should display a number of sessions every 15 minutes . and the group by clause will remain the same (ie. every minute) For example : at 10:00:00 it should represent the number of sessions (sessionID) in last 15 minutes to 10:00:00 ie: 9:45:00 to 10:00:00 and so on

This one gets a little complicated. First you need to create a TimeSlot table that breaks a day into 15 minute chunks.

CREATE TABLE [TimeSlot](
    [ID] [int] IDENTITY(1,1) NOT NULL,
    [BeginTime] [datetime] NULL,
    [EndTime] [datetime] NULL
) ON [PRIMARY]

To fill the table:

DECLARE @Start DATETIME
SET @Start='00:00:00'

WHILE (@Start <'23:59:00')
BEGIN
    INSERT INTO TimeSlot(BeginTime, EndTime) values (@Start,dateadd(mi,15,@Start))
    SET @Start=dateadd(mi,15,@Start)
END

Then, for the ugly SQL:

SELECT  convert(nvarchar(8), L.TimeLog,112) + ' ' +  convert(nvarchar(8), S.BeginTime, 108) AS SlotBeginTime ,Count(LogID) AS Sessions
FROM  LogData L, TimeSlot S
WHERE (convert(nvarchar(8), L.TimeLog,112) +  convert(nvarchar(8), L.TimeLog, 108))  >= (convert(nvarchar(8), L.TimeLog,112)+ convert(nvarchar(8), S.BeginTime,108)) 
  AND (convert(nvarchar(8), L.TimeLog,112) +  convert(nvarchar(8), L.TimeLog, 108))  < (convert(nvarchar(8), L.TimeLog,112)+ convert(nvarchar(8), S.EndTime,108))
GROUP BY convert(nvarchar(8), L.TimeLog,112) + ' ' +  convert(nvarchar(8), S.BeginTime, 108) 
ORDER BY SlotBeginTime;

Try it out. I didn't check how well it works across multiple dates but it does would for a single date.