说明：  This document explains the format of the MDB files. MDB files store the actual contents of a database record. The MDB file begins with a variable length header describing the overall database configuration. The header is followed by one or more variable length records. Every MDB file must have (generally as its first record, though it is not clear if this location is a requirement) a null record. This record is used by parent databases, if they exist, to specify a null value for one of their access keys.