1. 분으로 GROUP BY를 할 수 있게 CASE문 사용하여 기준값  지정 또는 날짜 지정
2. 위 지정값으로 GROUP BY

SELECT issue_date
   , SUM(var1) / COUNT(Var1)
   , SUM(var2) / COUNT(Var2)
   , SUM(var3) / COUNT(Var3)
FROM (
   SELECT
      /*
      grouptarget = case
            when datepart(mi,issue_date) >= 0 AND datepart(mi,issue_date) <= 5 then 1
            when datepart(mi,issue_date) >= 6 AND datepart(mi,issue_date) <= 10 then 2
            when datepart(mi,issue_date) >= 11 AND datepart(mi,issue_date) <= 15 then 3
            when datepart(mi,issue_date) >= 16 AND datepart(mi,issue_date) <= 20 then 4
            when datepart(mi,issue_date) >= 21 AND datepart(mi,issue_date) <= 25 then 5
            when datepart(mi,issue_date) >= 26 AND datepart(mi,issue_date) <= 30 then 6
            when datepart(mi,issue_date) >= 31 AND datepart(mi,issue_date) <= 35 then 7
            when datepart(mi,issue_date) >= 36 AND datepart(mi,issue_date) <= 40 then 8
            when datepart(mi,issue_date) >= 41 AND datepart(mi,issue_date) <= 45 then 9
            when datepart(mi,issue_date) >= 46 AND datepart(mi,issue_date) <= 50 then 10
            when datepart(mi,issue_date) >= 51 AND datepart(mi,issue_date) <= 55 then 11
            when datepart(mi,issue_date) >= 56 AND datepart(mi,issue_date) <= 60 then 12
           end
      */
      issue_date = case
            when datepart(mi,issue_date) >= 0 AND datepart(mi,issue_date) <= 5 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':00:00')
            when datepart(mi,issue_date) >= 6 AND datepart(mi,issue_date) <= 10 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':06:00')
            when datepart(mi,issue_date) >= 11 AND datepart(mi,issue_date) <= 15 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':11:00')
            when datepart(mi,issue_date) >= 16 AND datepart(mi,issue_date) <= 20 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':16:00')
            when datepart(mi,issue_date) >= 21 AND datepart(mi,issue_date) <= 25 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':21:00')
            when datepart(mi,issue_date) >= 26 AND datepart(mi,issue_date) <= 30 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':26:00')
            when datepart(mi,issue_date) >= 31 AND datepart(mi,issue_date) <= 35 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':31:00')
            when datepart(mi,issue_date) >= 36 AND datepart(mi,issue_date) <= 40 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':36:00')
            when datepart(mi,issue_date) >= 41 AND datepart(mi,issue_date) <= 45 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':41:00')
            when datepart(mi,issue_date) >= 46 AND datepart(mi,issue_date) <= 50 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':46:00')
            when datepart(mi,issue_date) >= 51 AND datepart(mi,issue_date) <= 55 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':51:00')
            when datepart(mi,issue_date) >= 56 AND datepart(mi,issue_date) <= 59 then CONVERT(DATETIME, CONVERT(CHAR(10), issue_date, 121) + ' ' + CONVERT(CHAR(2), DATEPART(hh, issue_date)) + ':56:00')
           end
     , var1
     , var2
     , var3
   FROM DBO.TEST
) AS A
GROUP BY issue_date

무식한 방법이죠 ^^;; 이것은 분으로 그룹핑을 한것이고, 날짜별로도 필요하다면 여기서 한번더 그룹핑을 하시면 될듯합니다.

IF OBJECT_ID('TESTTB') IS NOT NULL

       DROP TABLE TESTTB

CREATE TABLE TESTTB

       ( IDX INT IDENTITY(1,1)

       , ISSUE_DATE DATETIME

       , VAR1 NUMERIC(5, 2)

       , VAR2 NUMERIC(5, 2)

       , VAR3 NUMERIC(5, 2))

GO

INSERT INTO TESTTB

       SELECT '2007-11-19 12:12:13', 18.1,     2      ,  45.7  UNION ALL

       SELECT '2007-11-19 12:13:14', 18.3,     1.9,   45.6  UNION ALL

       SELECT '2007-11-19 12:14:15', 18.2,     2.1,   45.8  UNION ALL

       SELECT '2007-11-19 12:15:16', 18.3,     2      ,  45.7  UNION ALL

       SELECT '2007-11-19 12:16:13', 18.1,     2      ,  45.7  UNION ALL

       SELECT '2007-11-19 12:17:14', 18.3,     1.9,   45.6  UNION ALL

       SELECT '2007-11-19 12:18:15', 18.2,     2.1,   45.8  UNION ALL

       SELECT '2007-11-19 12:19:16', 18.3,     2      ,  45.7  UNION ALL

       SELECT '2007-11-19 12:20:13', 18.1,     2      ,  45.7  UNION ALL

       SELECT '2007-11-19 12:21:14', 18.3,     1.9,   45.6  UNION ALL

       SELECT '2007-11-19 12:22:15', 18.2,     2.1,   45.8  UNION ALL

       SELECT '2007-11-19 12:23:16', 18.3,     2      ,  45.7  UNION ALL

       SELECT '2007-11-19 12:30:13', 18.1,     2      ,  45.7  UNION ALL

       SELECT '2007-11-19 12:50:14', 18.3,     1.9,   45.6  UNION ALL

       SELECT '2007-11-19 12:55:15', 18.2,     2.1,   45.8  UNION ALL

       SELECT '2007-11-19 12:56:16', 18.3,     2      ,  45.7  UNION ALL

       SELECT '2007-11-19 12:24:13', 18.1,     2      ,  45.7  UNION ALL

       SELECT '2007-11-19 12:36:14', 18.3,     1.9,   45.6  UNION ALL

       SELECT '2007-11-19 12:42:15', 18.2,     2.1,   45.8 

GO    

SELECT

       A.ISSUE_DATE

       , B.AVG_VAR1

       , B.AVG_VAR2

       , B.AVG_VAR3

FROM

       TESTTB A

       INNER JOIN (

                           SELECT

                                   IDX = MIN(IDX)

                                 , AVG_VAR1 = AVG(VAR1)

                                 , AVG_VAR2 = AVG(VAR2)

                                 , AVG_VAR3 = AVG(VAR3)

                           FROM

                                 TESTTB

                           GROUP BY DATEPART(MI, ISSUE_DATE) /5

       ) B

       ON A.IDX = B.IDX

GO