What's the 1991 census of the city of Carpi?
CREATE TABLE "references" (
  "num" real,
  "city" text,
  "1981_census" real,
  "1991_census" real,
  "2001_census" real,
  "2010_est" real,
  "region" text
);
SELECT MAX("1991_census") FROM "references" WHERE "city"='Carpi';