1. LEFT JOIN/IS NULL

DELETE b FROM tbl
  LEFT JOIN rel ON rel.id = tbl.fileid 
      WHERE rel.id IS NULL

2. NOT EXISTS

DELETE FROM tbl 
 WHERE NOT EXISTS(SELECT NULL
                    FROM rel
                   WHERE rel.id = tbl.fileid)

3. NOT IN

DELETE FROM tbl
 WHERE tbl.fileid NOT IN (SELECT rel.id 
                        FROM rel)