What is the roll number of Poroti school, which has a 6 decile?
CREATE TABLE "whangarei" (
  "name" text,
  "years" text,
  "area" text,
  "authority" text,
  "decile" text,
  "roll" text
);
SELECT "roll" FROM "whangarei" WHERE "decile"='6' AND "name"='poroti school';