SELECT  CONVERT(VARCHAR(10),TIME,120) AS DATE
      ,DATEPART(hh, TIME) AS hh,
    DATEPART(mi, TIME)/30*30 AS mm,
    FIRST(opn)
        ,MAX(hi) AS hi
        ,MIN(lo) AS lo
    ,LAST(clo)
    ,SUM(vol)
 FROM maindata WHERE symbol='nifty'
GROUP BY CONVERT(VARCHAR(10),TIME,120),DATEPART(hh, TIME),DATEPART(mi, TIME)/30*30