What is the highest Laps, when Bike is "Yamaha YZF-R1", when Rider is "David Checa", and when Grid is greater than 18?
CREATE TABLE "superbike_race_2_classification" (
  "rider" text,
  "bike" text,
  "laps" real,
  "time" text,
  "grid" real
);
SELECT MAX("laps") FROM "superbike_race_2_classification" WHERE "bike"='yamaha yzf-r1' AND "rider"='david checa' AND "grid">18;