.MDB

Introduction

An .mdb file is a database made by Microsoft Access, the desktop database in Microsoft Office. One file holds the tables and their data, and usually also the queries, forms and reports built on them. The storage engine behind it is called Jet.

Access 2007 introduced a newer version of the engine, ACE, and a new extension, .accdb, which adds features such as multi-valued fields, attachments and stronger encryption. Older Access versions cannot open .accdb files. The formats are proprietary; what is known of the page layout comes from the open-source MDB Tools and Jackcess projects.

Structure

The file is a series of fixed-size pages: 2,048 bytes in Jet 3 (Access 97) and 4,096 bytes in Jet 4 (Access 2000 to 2003) and ACE. The first page is the header. From byte 4 it holds the text Standard Jet DB in an .mdb file or Standard ACE DB in an .accdb file, and a version number follows. Much of the rest of the header is obscured with a fixed key.

Other pages hold table definitions, data rows, indexes and the map of which pages belong to which table. A system table, MSysObjects, lists every table, query, form and report in the file. While a database is open, Access keeps a lock file beside it (.ldb or .laccdb).

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