CREATE TABLE "250cc_classification" (
  "rider" text,
  "manufacturer" text,
  "laps" real,
  "time_retired" text,
  "grid" real
);

Question: What time/retired for a grid less than 19, under 21 laps, and made by aprilia?
SQL: SELECT "time_retired" FROM "250cc_classification" WHERE "grid"<19 AND "manufacturer"='aprilia' AND "laps"<21;

Question: What is the grid for dirk heidolf?
SQL: SELECT SUM("grid") FROM "250cc_classification" WHERE "rider"='dirk heidolf';