CREATE TABLE "clubs_and_venues" (
  "club" text,
  "location" text,
  "stadium" text,
  "capacity" real,
  "current_manager" text
);

Question: What is the capacity for the stadium of kadrioru stadium and a current manager of sergei ratnikov?
SQL: SELECT AVG("capacity") FROM "clubs_and_venues" WHERE "stadium"='kadrioru stadium' AND "current_manager"='sergei ratnikov';