A "--" in the district column means, that the town is a district-free town, i.e. it is by itself a district.
... has a total area of 23.8 km² (9.2 mi²). 23.5 km² (9.1 mi²) of it is land and 0.3 km² (0.1 mi²) of it is water. The total area is 1.09% ...