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SQL Server 2005 Express Database

Microsoft's SQL Server 2005 Express is the new, FREE, lightweight edition database based on Microsoft's core SQL Server 2005 technology. SQL Server 2005 Express Edition is a royalty-free, redistributable database engine that is fully compatible with SQL Server. It therefore provides a seamless upgrade path to the Workgroup, Standard and Enterprise Editions of SQL Server if an application grows beyond the storage and scalability limits of the Express Edition.

Identical to a SQL Server 2005 database.
Limited to 4 GByte Database Size
Limited to 1 CPU.
Supports only 1 GByte of Memory.

Note: Please review the following articles for more information on SQL 2005 Express:

Microsoft SQL Server 2005 Express Edition Home Page - Microsoft

Microsoft SQL Server 2005 Express Edition - Microsoft

Microsoft SQL Server 2005 Express Edition Overview - Microsoft

Microsoft SQL Server Express 2005 - Don Schlichting

SQL Server 2005 Options - eWeek