Table schema:
CREATE TABLE table_name_87 (market_value__us INTEGER, rank VARCHAR, revenues__us$_billion_ VARCHAR, company VARCHAR)

Question: 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?
SQL: SELECT AVG(market_value__us) AS $_billion_ FROM table_name_87 WHERE revenues__us$_billion_ > 6.2 AND company = "national bank of greece" AND rank < 1