Which Servedby has a Local authority [a ] of thurrock, and a Station of ockendon?
CREATE TABLE "stations" (
  "station" text,
  "local_authority_a" text,
  "servedby" text,
  "station_users_2004_05" real,
  "station_users_2005_06" real
);
SELECT "servedby" FROM "stations" WHERE "local_authority_a"='thurrock' AND "station"='ockendon';