CREATE TABLE table_204_576 (id DECIMAL, "year" DECIMAL, "best" TEXT, "location" TEXT, "date" TEXT, "world rank" TEXT)

did bekele have a better world ranking in 2012 or 2007 ?
SELECT "year" FROM table_204_576 WHERE "year" IN (2012, 2007) ORDER BY "world rank" LIMIT 1
SELECT "year" FROM "table_204_576" WHERE "year" IN (2012, 2007) ORDER BY "world rank" NULLS FIRST LIMIT 1