Oracle Analytic Function

SELECT
SALES_OFFICER_CODE
, SALES_OFFICER_NAME
, SOURCE_START_DATE
, EFFECTIVE_DATE
, END_DATE
, SOURCE_END_DATE
, ROW_NUMBER() OVER (PARTITION BY SALES_OFFICER_CODE ORDER BY SALES_OFFICER_CODE,SOURCE_START_DATE desc) as rn
, FIRST_VALUE(SOURCE_START_DATE) OVER (PARTITION BY SALES_OFFICER_CODE ORDER BY SALES_OFFICER_CODE,SOURCE_START_DATE desc) as fv
, NVL(LAG(SOURCE_START_DATE) OVER (PARTITION BY SALES_OFFICER_CODE ORDER BY SALES_OFFICER_CODE,SOURCE_START_DATE desc),to_date('1900/01/01','yyyy-mm-dd')) as lag
FROM SDMSYSDB.TSDMD_SALES_OFFICER_ROLE