CREATE TABLE table_204_255 (id NUMBER, "constituency number" NUMBER, "name" CLOB, "reserved for  ( sc/st/none ) " CLOB, "district" CLOB, "number of electorates  ( 2009 ) " NUMBER)

how many districts are there ?
SELECT COUNT(DISTINCT "district") FROM table_204_255
SELECT COUNT(DISTINCT "district") FROM "table_204_255"