What's the diameter of the 2006 equestrian Reverse?
CREATE TABLE "150_yuan" (
  "year" real,
  "denomination" text,
  "reverse" text,
  "weight" text,
  "diameter" text,
  "fineness" text,
  "series" text
);
SELECT "diameter" FROM "150_yuan" WHERE "year"=2006 AND "reverse"='equestrian';