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A data source is a source of data and the connection information needed to access that data. Examples of data sources are Microsoft Access, Microsoft SQL Server, Oracle RDBMS, a spreadsheet, and a text file. Examples of connection information include server location, database name, logon ID, password, and various ODBC driver options that describe how to connect to the data source.
In the ODBC architecture, an application (such as Access or a Microsoft Visual Basic program) connects to the ODBC Driver Manager, which in turn uses a specific ODBC driver (for example, Microsoft SQL ODBC driver) to connect to a data source (in this case, an SQL Server database). In Access, you use ODBC data sources to connect to data external to Access that do not have built-in drivers.
To connect to these data sources, you must do the following:
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Machine data sources store connection information in the Windows Registry on a specific computer with a user-defined name. You can use machine data sources only on the computer they are defined on. There are two types of machine data sources — user and system. User data sources can be used only by the current user and are visible only to that user. System data sources can be used by all users on a computer and are visible to all users on the computer and system-wide services, such as Microsoft Windows NT services. A machine data source is especially useful when you want to provide added security, because only users who are logged on can view a machine data source and it cannot be copied by a remote user to another computer.
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File data sources (also called DSN files) store connection information in a text file, not the Windows Registry, and are generally more flexible to use than machine data sources. For example you can copy a file data source to any computer with the correct ODBC driver so that your application can rely on consistent and accurate connection information to all the computers it uses. Or you can place the file data source on a single server, share it between many computers on the network, and easily maintain the connection information in one location.
A file data source can also be unshareable. An unshareable file data source resides on a single computer and points to a machine data source. You can use unshareable file data sources to access existing machine data sources from file data sources.
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If you are a programmer, you can define a formatted connect string in your Visual Basic code that specifies the connection information. Using a connect string avoids defining a machine or file DSN and passes the connection information directly to the ODBC Driver Manager. This is useful, for example, when you want to avoid requiring system administrators or users from first creating a DSN, or to simplify the installation of your application.
For more information about programming ODBC, see the ODBC 3.0 Programmer's Reference Manual.
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