What is the average rebounds for the player who started in 1986 and who plays SG?
CREATE TABLE "g" (
  "player" text,
  "pos" text,
  "from" real,
  "school_country" text,
  "rebs" real,
  "asts" real
);
SELECT AVG("rebs") FROM "g" WHERE "from"=1986 AND "pos"='sg';