case when  (D.C_ZONE is null or D.C_ZONE = '')
then
(coalesce(DL.C_ZONE, ' ')  as C_ZONE from S.SNFDLR DL)
else
(coalesce(D.C_ZONE, ' ' )  as C_ZONE from S.SFDDIST D) end
    LEFT OUTER join S.SRCZNRG Z ON ( DL.C_ZONE=Z.C_ZONE )
    LEFT OUTER join S.SRCZNRG Z ON ( D.C_ZONE=Z.C_ZONE )