CREATE TABLE table_204_579 (id NUMBER, "code" CLOB, "district" CLOB, "headquarters" CLOB, "population  ( as of 2011 ) " NUMBER, "area  ( km2 ) " NUMBER, "density  ( /km2 ) " NUMBER)

tell me a district that only has a density of 51 .
SELECT "district" FROM table_204_579 WHERE "density (/km2)" = 51
SELECT "district" FROM "table_204_579" WHERE "density (/km2)" = 51