| title | description | author | ms.author | ms.reviewer | ms.date | ms.service | ms.topic | helpviewer_keywords | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Linked Servers (Database Engine) |
Linked Servers (Database Engine) |
WilliamDAssafMSFT |
wiassaf |
vanto |
09/23/2024 |
sql |
conceptual |
|
[!INCLUDE SQL Server SQL MI]
Linked servers enable the [!INCLUDE ssDEnoversion] and [!INCLUDE ssazuremi] to read data from the remote data sources and execute commands against the remote database servers (for example, OLE DB data sources) outside of the instance of [!INCLUDE ssNoVersion]. Typically linked servers are configured to enable the [!INCLUDE ssDE] to execute a [!INCLUDE tsql] statement that includes tables in another instance of [!INCLUDE ssNoVersion], or another database product such as Oracle. Many types OLE DB data sources can be configured as linked servers, including third-party database providers and Azure Cosmos DB.
Note
Linked servers are available in [!INCLUDE ssNoVersion] and [!INCLUDE ssazuremi] (with some constraints). Linked servers are not available in [!INCLUDE ssazure-sqldb].
Linked servers enable you to implement distributed databases that can fetch and update data in other databases. Linked servers are a good solution in the scenarios where you need to implement database sharding without need to create a custom application code or directly load from remote data sources. Linked servers offer the following advantages:
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The ability to access data from outside of [!INCLUDE ssNoVersion].
-
The ability to issue distributed queries, updates, commands, and transactions on heterogeneous data sources across the enterprise.
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The ability to address diverse data sources similarly.
You can configure a linked server by using [!INCLUDE ssManStudioFull] or by using the sp_addlinkedserver statement. OLE DB providers vary greatly in the type and number of parameters required. For example, some providers require you to provide a security context for the connection using sp_addlinkedsrvlogin. Some OLE DB providers allow [!INCLUDE ssNoVersion] to update data on the OLE DB source. Others provide only read-only data access. For information about each OLE DB provider, consult documentation for that OLE DB provider.
A linked server definition specifies the following objects:
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An OLE DB provider
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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 various files and file formats. These include text files, spreadsheet data, and the results of full-text content searches.
Starting with [!INCLUDE sql-server-2019], the Microsoft OLE DB Driver for SQL Server (PROGID: MSOLEDBSQL) is the default OLE DB provider. In earlier versions, the SQL Server Native Client (PROGID: SQLNCLI11) was the default OLE DB provider.
Important
[!INCLUDE snac-removed-oledb-only]
Linked servers to [!INCLUDE msCoName] Access and Excel sources are only supported by Microsoft when using the 32-bit Microsoft.JET.OLEDB.4.0 OLE DB provider.
Note
[!INCLUDE ssNoVersion] distributed queries are designed to work with any OLE DB provider that implements the required OLE DB interfaces. However, [!INCLUDE ssNoVersion] has been tested against the default OLE DB provider.
The following illustration shows the basics of a linked server configuration.
:::image type="content" source="media/linked-servers-database-engine/lsvr.gif" alt-text="Diagram showing 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, [!INCLUDE ssNoVersion] parses the command and sends requests to OLE DB. The rowset request might be in the form of executing a query against the provider or opening a base table from the provider.
Note
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 [!INCLUDE ssNoVersion].
Important
When an OLE DB provider is used, the account under which the [!INCLUDE ssNoVersion] service runs must have read and execute permissions for the directory, and all subdirectories, in which the provider is installed. This includes Microsoft-released providers, and any third-party providers.
Note
Linked servers support Active Directory pass-through authentication when using full delegation. Starting with [!INCLUDE ssSQL17] CU17, pass-through authentication with constrained delegation is also supported; however, resource-based constrained delegation is not supported.
There's a set of options that control how [!INCLUDE ssNoVersion] loads and uses OLE DB providers that are specified in the registry.
When you're setting up a linked server, register the connection information and data source information with [!INCLUDE ssNoVersion]. After being 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:
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Create a linked server definition by running
sp_addlinkedserver. -
View information about the linked servers defined in a specific instance of [!INCLUDE ssNoVersion] by running a query against the
sys.serverssystem catalog view. -
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 [!INCLUDE ssManStudioFull]. 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.
References to temporary objects will always resolve to the local instance's tempdb where applicable, even when prefixing with the linked server name.
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.
Azure SQL Managed Instance linked servers support both SQL authentication and authentication with Microsoft Entra ID (formerly Azure Active Directory).
To use SQL Agent jobs on Azure SQL Managed Instance to query a remote server through a linked server, use sp_addlinkedsrvlogin to create a mapping from a login on the local server to a login on the remote server. When the SQL Agent job connects to the remote server through the linked server, it executes the T-SQL query in the context of the remote login. For more information, see SQL Agent jobs with Azure SQL Managed Instance.
Two supported Microsoft Entra authentication modes are: managed identity and pass-through. Managed identity authentication can be used to allow local logins to query remote linked servers. Pass-through authentication allows a principal that can authenticate with a local instance to access a remote instance via a linked server.
To use Microsoft Entra pass-through authentication for a linked server in Azure SQL Managed Instance, you need the following prerequisites:
- The same principal is added as a login on the remote server.
- Both instances are members of the SQL trust group.
Note
Existing definitions of linked servers that were configured for pass-through mode support Microsoft Entra authentication. The only requirement for this would be to add SQL Managed Instance to the Server Trust Group.
The following limitations apply to Microsoft Entra authentication for linked servers in Azure SQL Managed Instance:
- Microsoft Entra authentication isn't supported for SQL managed instances in different Microsoft Entra tenants.
- Microsoft Entra authentication for linked servers is supported only with OLE DB driver version 18.2.1 and higher.
Currently, MSOLEDBSQL19 prevents the creation of linked servers without encryption and a trusted certificate (a self-signed certificate is insufficient). If linked servers are required, use the existing supported version of MSOLEDBSQL.