select  case
 when(z.za_data_rz is null) then z.za_data
 when(z.za_godz_rz is null) then z.za_godz
 end,
  z.nr_zlecenia,z.za_data,t.nr_rej,t.kierowca1, z.ZA_DATA_RZ,z.wy_data_rz from zlecenia z
inner join tabor t on (t.ID_TABORU=z.SAMOCHOD_ID)
 where z.za_data between '2018-08-15' and '2018-09-15'