UPDATE PL_REQUEST_DOC SET REQ_DT = EFFEC_DT WHERE ISNULL(IS_BACKDAY,0)=1 OR CONVERT(DATE,EFFEC_DT,103) < CONVERT(DATE,CREATE_DT,103)

UPDATE PL_REQUEST_DOC SET REQ_DT = CREATE_DT WHERE ISNULL(IS_BACKDAY,0)=0 OR CONVERT(DATE,EFFEC_DT,103) >= CONVERT(DATE,CREATE_DT,103)