What is the Date that has MARTIN of 7–9% and no Undecided?
CREATE TABLE "opinion_polls" (
  "source" text,
  "date" text,
  "martin" text,
  "gr_ne" text,
  "others" text,
  "undecided" text
);
SELECT "date" FROM "opinion_polls" WHERE "undecided"='–' AND "martin"='7–9%';