Få rangering af en række i mysql-forespørgsel

Fejlen er ret tydelig. Hver afledt tabel skal have sit eget alias. Du skal alias (SELECT @rownum := 0) sådan:

SELECT 
  @rownum := @rownum + 1 AS rank, 
  name,
  vote 
FROM table, (SELECT @rownum := 0) t --This what you were missing an alias
ORDER BY vote DESC

SQL Fiddle Demo