DB Error: unknown error

SELECT count(newsID) as count, ch99_volley_category.* FROM ch99_volley_category,ch99_volley_news WHERE ch99_volley_category.categoryID=ch99_volley_news.categoryID AND parentID=0 GROUP BY categoryID ORDER BY count DESC , ch99_volley_category.categoryID [nativecode=1 ** Can't create/write to file '/tmp/#sql_cbc_0.MYI' (Errcode: 30)]