Du kan använda LAG funktion:
SELECT
"REGION"
, "COUNTRY"
, "IDNum"
, "CUSTOMER"
, "BUSSINESS"
, "FISCALYEARMONTH"
, "FISCALYEAR"
, SUM("VALUE")
, MAX(CASE WHEN to_date(prev_fym,'YYYYMM') >= ADD_MONTHS (to_date("FISCALYEARMONTH",'YYYYMM'), -24) THEN 'Existing'
ELSE NULL END) "HISTORICAL_PURCHASE_FLAG"
FROM
(
SELECT
T1."REGION"
, T1."COUNTRY"
, T1."IDNum"
, T1."CUSTOMER"
, T1."BUSSINESS"
, T3."FISCALYEARMONTH"
, T3."FISCALYEAR"
, T4."VALUE"
, LAG ("FISCALYEARMONTH", 1) OVER (PARTITION BY T1."IDNum" ORDER BY T3."FISCALYEARMONTH" DESC) prev_fym
FROM
"DATABASE"."SALES" T4
, "DATABASE"."CUSTOMER" T1
, "DATABASE"."PRODUCT" T2
, "DATABASE"."TIME" T3
WHERE
T4."CUSTOMERID" = T1."CUSTOMERID"
AND T4."PRODUCTID" = T2."PRODUCTID"
AND T4."DATEID" = T3."DATEID"
AND T1."COUNTRY" IN ('ENGLAND', 'France')
AND T3."FISCALYEAR" IN ('2014','2015','2016')
)
WHERE "FISCALYEAR" IN ('2016')
GROUP BY
"REGION"
, "COUNTRY"
, "IDNum"
, "CUSTOMER"
, "BUSSINESS"
, "FISCALYEARMONTH"
, "FISCALYEAR"
;