What is the abbreviation of the district of Mahbubnagar?
CREATE TABLE "external_links" (
  "code" text,
  "district" text,
  "headquarters" text,
  "population_2011" real,
  "area_km" real,
  "density_km" real,
  "official_website" text
);
SELECT "code" FROM "external_links" WHERE "district"='Mahbubnagar';