I'm not even sure you need a row number function. You appear to just be selecting the distinct values for the CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, and ENDDATE, which can allow for use of either DISTINCT or GROUP BY.

Here's the code:

DECLARE @HISTORY AS TABLE (

CUSTOMER VARCHAR(10),

PLANNBR VARCHAR(10),

SUBPLAN VARCHAR(3),

STARTDATE DATETIME,

ENDDATE DATETIME,

HISTORYMONTH VARCHAR(6)

)

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111111', 'H1111LAC', '006', '2014-01-01', '2014-05-31', '201401')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111111','H1111LAC', '006', '2014-01-01', '2014-05-31', '201402')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111111','H1111LAC', '006', '2014-01-01', '2014-05-31', '201403')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111111','H1111LAC', '006', '2014-01-01', '2014-05-31', '201404')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111111','H1111LAC', '006', '2014-01-01', '2014-05-31', '201405')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111111','H1111OC', '010', '06-01-2014', '2999-12-31', '201406')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111112', 'H1111LAC', '006', '2014-01-01', '2014-05-31', '201401')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111112','H1111LAC', '006', '2014-01-01', '2014-05-31', '201402')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111112','H1111LAC', '006', '2014-01-01', '2014-05-31', '201403')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111112','H1111LAC', '006', '2014-01-01', '2014-05-31', '201404')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111112','H1111LAC', '006', '2014-01-01', '2014-05-31', '201405')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111112','H1111LAC', '018', '06-01-2014', '2999-12-31', '201406')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111113', 'H1111LAC', '006', '2014-01-01', '2999-12-31', '201401')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111113','H1111LAC', '006', '2014-01-01', '2999-12-31', '201402')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111113','H1111LAC', '006', '2014-01-01', '2999-12-31', '201403')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111113','H1111LAC', '006', '2014-01-01', '2999-12-31', '201404')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111113','H1111LAC', '006', '2014-01-01', '2999-12-31', '201405')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111113','H1111LAC', '006', '2014-01-01', '2999-12-31', '201406')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111114','H1111LAC', '006', '2014-01-01', '2014-02-28', '201402')

INSERT INTO @HISTORY(CUSTOMER, PLANNBR, SUBPLAN, STARTDATE, ENDDATE, HISTORYMONTH) VALUES('9111114','H1111LAC', '006', '2014-04-01', '2999-12-31', '201404')

SELECT H.CUSTOMER, H.PLANNBR, H.SUBPLAN, H.STARTDATE, H.ENDDATE

FROM @HISTORY AS H

GROUP BY H.CUSTOMER, H.PLANNBR, H.SUBPLAN, H.STARTDATE, H.ENDDATE