What is the link for january 31 – february 3, 2011?
CREATE TABLE "opinion_polls" (
  "polling_firm" text,
  "date_of_polling" text,
  "link" text,
  "liberal" real,
  "green" real
);
SELECT "link" FROM "opinion_polls" WHERE "date_of_polling"='january 31 – february 3, 2011';