What is the average market value with a revenue greater than 6.2, a Company of national bank of greece, and a Rank smaller than 1?
CREATE TABLE "forbes_global_2000" (
  "rank" real,
  "company" text,
  "revenues_us_billion" real,
  "profit_us_billion" real,
  "assets_us_billion" real,
  "market_value_us_billion" real
);
SELECT AVG("market_value_us_billion") FROM "forbes_global_2000" WHERE "revenues_us_billion">6.2 AND "company"='national bank of greece' AND "rank"<1;