Refined and resolved !!!

Thanks for all your input.

This is the working query.

SELECT

YEAR(CAST (a.ENDDATE AS DATETIME)),

MONTH(CAST (a.ENDDATE AS DATETIME)),

(

SELECT

SUM(b.DEPTH)

FROM

HOLELOCATION b

WHERE

b.HOLEID LIKE 'RHRC%'

AND ( CAST(YEAR(CAST(b.ENDDATE AS DATETIME)) AS VARCHAR) + RIGHT('00'+ CAST( MONTH(CAST (b.ENDDATE AS DATETIME)) AS VARCHAR(2)),2)) <= ( CAST(YEAR(CAST(a.ENDDATE AS DATETIME)) AS VARCHAR) + RIGHT('00'+ CAST( MONTH(CAST (a.ENDDATE AS DATETIME)) AS VARCHAR(2)),2))

AND YEAR(CAST (b.ENDDATE AS DATETIME)) IS NOT NULL AND MONTH(CAST (b.ENDDATE AS DATETIME)) IS NOT NULL

) AS DEPTH

FROM

HOLELOCATION a

WHERE a.HOLEID LIKE 'RHRC%' AND a.ENDDATE IS NOT NULL

GROUP BY

YEAR(CAST (a.ENDDATE AS DATETIME)),

MONTH(CAST (a.ENDDATE AS DATETIME))

ORDER BY

YEAR(CAST (a.ENDDATE AS DATETIME)) ASC,

MONTH(CAST (a.ENDDATE AS DATETIME)) ASC