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Linking Servers

A linked server configuration enables SQL Server to execute commands against OLE DB data sources on remote servers. Linked servers offer the following advantages:

  • Remote server access.

  • The ability to issue distributed queries, updates, commands, and transactions on heterogeneous data sources across the enterprise.

  • The ability to address diverse data sources similarly.

A linked server definition specifies the following objects:

  • An OLE DB provider

  • An OLE DB data source

An OLE DB provider is a DLL that manages and interacts with a specific data source. An OLE DB data source identifies the specific database that can be accessed through OLE DB. Although data sources queried through linked server definitions are ordinarily databases, OLE DB providers exist for a variety of files and file formats. These include text files, spreadsheet data, and the results of full-text content searches.

The Microsoft SQL Server Native Client OLE DB Provider (PROGID: SQLNCLI10) is the official OLE DB provider for SQL Server.

NoteNote

SQL Server distributed queries are designed to work with any OLE DB provider that implements the required OLE DB interfaces. However, SQL Server has been tested against only the SQL Server Native Client OLE DB Provider and certain other providers. For more information, see OLE DB Providers Tested with SQL Server.

The following illustration shows the basics of a linked server configuration.

Client tier, server tier, and database server tier

Typically, linked servers are used to handle distributed queries. When a client application executes a distributed query through a linked server, SQL Server parses the command and sends requests to OLE DB. The rowset request may be in the form of executing a query against the provider or opening a base table from the provider.

For a data source to return data through a linked server, the OLE DB provider (DLL) for that data source must be present on the same server as the instance of SQL Server.

When a third-party OLE DB provider is used, the account under which the SQL Server service runs must have read and execute permissions for the directory, and all subdirectories, in which the provider is installed.

There is a set of options that control how SQL Server loads and uses OLE DB providers that are specified in the registry. For more information, see Configuring OLE DB Providers for Distributed Queries.

When you are setting up a linked server, register the connection information and data source information with SQL Server. After registered, that data source can be referred to with a single logical name.

You can use stored procedures and catalog views to manage linked server definitions:

  • Create a linked server definition by running sp_addlinkedserver.

  • View information about the linked servers defined in a specific instance of SQL Server by running a query against the sys.servers system catalog views.

  • Delete a linked server definition by running sp_dropserver. You can also use this stored procedure to remove a remote server.

You can also define linked servers by using SQL Server Management Studio. In the Object Explorer, right-click Server Objects, select New, and select Linked Server. You can delete a linked server definition by right-clicking the linked server name and selecting Delete.

When you execute a distributed query against a linked server, include a fully qualified, four-part table name for each data source to query. This four-part name should be in the form linked_server_name.catalog.schema.object_name. For more information, see Distributed Queries.

NoteNote

Linked servers can be defined to point back (loop back) to the server on which they are defined. Loopback servers are most useful when testing an application that uses distributed queries on a single server network. Loopback linked servers are intended for testing and are not supported for many operations, such as distributed transactions.

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