CREATE TABLE table_204_851 (id NUMBER, "serial number" NUMBER, "river" CLOB, "name" CLOB, "area  ( km2 ) " NUMBER, "district" CLOB, "co-ordinate" CLOB, "altitude  ( m ) " NUMBER)

what is the name of the river listed before chalakkudy ?
SELECT "river" FROM table_204_851 WHERE id = (SELECT id FROM table_204_851 WHERE "river" = 'chalakkudy') - 1
SELECT "river" FROM "table_204_851" WHERE "id" = (SELECT "id" FROM "table_204_851" WHERE "river" = 'chalakkudy') - 1