What class has less than 12 numbers built operated by southern?
CREATE TABLE "fleet_details" (
  "class" text,
  "operator" text,
  "no_built" real,
  "year_built" text,
  "cars_per_set" real,
  "unit_nos" text
);
SELECT "class" FROM "fleet_details" WHERE "no_built"<12 AND "operator"='southern';