Hello Dev, welcome to this comprehensive guide on SQL Server Linked Servers. The ability to connect and manage data sources is a vital aspect of SQL Server administration for today’s data-driven organizations. This article will take you through every aspect of SQL Server Linked Servers, exploring how to connect a range of data sources, including other SQL Server instances, Oracle, MySQL, and more, and how to use Linked Servers to query data across heterogeneous sources. Let’s dive in!
Introduction to SQL Server Linked Servers
SQL Server Linked Server allows you to connect and query data from other data sources, including other SQL Server instances, Oracle, MySQL, and more, using Transact-SQL (T-SQL), which is the native language of SQL Server. Linked Servers provide a way to treat all data sources as one by registering and providing a name for each data source, which can then be used to access and query data from the same SQL Server instance or even remotely via a network. If you have a requirement to query data from multiple data sources as if it were in one place, Linked Servers is the answer.
What is a Linked Server?
A Linked Server is essentially a database object that allows you to access and query data from a remote data source using T-SQL, as if it were a local database. A Linked Server can be created for different data sources, including SQL Server instances, Oracle, MySQL, Excel spreadsheets, and even flat files. Once a Linked Server is created, you can easily use four-part naming convention to execute queries against the remote data source by using the Linked Server’s name and the table or view name.
For example, if you have a Linked Server called “ORACLEDB” and you want to query a table called “CUSTOMERS” in the Oracle database, you can use the following T-SQL code:
| SELECT * | FROM ORACLEDB..SCOTT.CUSTOMERS; |
Why Use Linked Servers?
The main reason to use Linked Servers is to get data from multiple data sources as you were working with a single data source. This functionality allows you to build complex queries that join tables and data from multiple data sources without any need for ETL or data replication processes. Linked servers also allow you to access data from external data sources that are not natively supported by SQL Server.
Another advantage of Linked Servers is that they allow you to use the native security features of the data source. For example, when you create a Linked Server to an Oracle database, you can use Oracle’s native authentication and authorization mechanisms to secure access to the data source.
Creating a Linked Server in SQL Server
The process of creating a Linked Server in SQL Server is straightforward. You can do it either by using SQL Server Management Studio (SSMS) GUI interface or by executing a T-SQL statement. Here is an example of how to create a Linked Server to an Oracle database using T-SQL:
| EXEC sp_addlinkedserver | ‘ORACLEDB’, ‘Oracle’, | ‘MSDAORA’, | ‘xxxxxxx’, | NULL, | ‘xxxxxxx’; |
In this example, ‘ORACLEDB’ is the Linked Server’s name, ‘Oracle’ is the remote data source name, ‘MSDAORA’ is the provider name for the Oracle data source, and ‘xxxxxxx’ represents the Oracle username and password to connect to the Oracle database.
Connecting SQL Server to SQL Server Instance
When it comes to establishing a Linked Server to a SQL Server instance, the process is much simpler than establishing a Linked Server to another data source. Let’s explore how to establish a Linked Server to a SQL Server instance step by step.
Creating a Linked Server to Another SQL Server Instance
Creating a Linked Server to another SQL Server instance involves two steps:
- Defining a remote server
- Creating a Linked Server
The first step is to define the remote server by using sp_addserver stored procedure. Here is an example of how to define a remote server:
| EXEC sp_addserver | ‘REMOTESERVER’, ‘local’; |
In this example, ‘REMOTESERVER’ is the name of the remote SQL Server instance that you want to connect to from your local SQL Server instance. Setting the second parameter to ‘local’ means that the remote server is accessible locally.
The second step is to create a Linked Server to the remote SQL Server instance using sp_addlinkedserver stored procedure. Here is an example of how to create a Linked Server to the remote SQL Server Instance:
| EXEC sp_addlinkedserver | ‘LinkedServerName’, null, | ‘SQLNCLI’, null, | ‘Data Source=REMOTESERVER;User ID=username;Password=password’; |
In this example, ‘LinkedServerName’ is the name that you want to give to the Linked Server. ‘REMOTESERVER’ is the name of the remote SQL Server instance that you defined in step one, and ‘username’ and ‘password’ are the credentials that you need to provide for the Linked Server to access the remote SQL Server instance.
Connecting SQL Server to Oracle
Connecting SQL Server to Oracle is a bit more complicated than connecting to another SQL Server instance. Let’s explore how to establish a Linked Server to Oracle step by step.
Installing Oracle Client on SQL Server
To establish a Linked Server to Oracle, you need to have Oracle Client installed on the SQL Server. Here is the process of installing the Oracle Client on SQL Server:
- Download the Oracle Client from the Oracle official website
- Run the setup file and follow the installation prompts
- Choose the “Administrator” installation type and “Custom” installation type
- Select “Oracle Data Provider for .NET” and “Oracle Providers for ASP.NET” options
- Enter your Oracle Home name and click next to continue the installation process
- Select the installation folder on the SQL Server
- Click Install to begin the installation process
Creating a System Data Source Name (DSN)
After installing Oracle Client on SQL Server, you need to create a System DSN to connect to the Oracle database.
Here is the process of creating a System DSN:
- Open the ODBC Data Source Administrator, which can be found in the Control Panel in Windows
- Select the “System DSN” tab and click the “Add” button
- Select the “Oracle in OraClientXXg_home1” driver, where XX is the version of Oracle Client that you installed on SQL Server
- Enter the name of the data source in the “Data Source Name” field
- Enter the Oracle database name in the “TNS Service Name” field
- Enter the username and password for the Oracle database
- Click the “Test Connection” button to verify that the connection to the Oracle database is successful
- Click the “OK” button to save the System DSN
Creating a Linked Server to Oracle
After installing Oracle Client and creating a System DSN, you can now create a Linked Server to Oracle.
Here is an example of the T-SQL code required to create a Linked Server to Oracle:
| EXEC sp_addlinkedserver | ‘LinkedServerName’, null, | ‘MSDAORA’, | ‘Data Source=OracleDB;User ID=username;Password=password’ |
In this example, ‘LinkedServerName’ is the name that you want to give to the Linked Server. The ‘OracleDB’ is the name of the System DSN that you created in the previous step. ‘username’ and ‘password’ are the credentials that you need to provide for the Linked Server to access the Oracle database.
Querying Data with Linked Servers
Now that we have set up Linked Servers to SQL Server and Oracle, let’s explore how to query data using Linked Servers.
Executing Queries with Four-Part Naming Convention
The easiest way to query data from the remote data source is to use the four-part naming convention. Here is an example of how to query data from an Oracle database using Linked Server:
| SELECT * | FROM LinkedServerName..SCOTT.CUSTOMERS; |
In this example, ‘SCOTT’ is the Oracle user who owns the ‘CUSTOMERS’ table that we want to query. The ‘LinkedServerName’ is the name of the Linked Server that we created in the previous step.
Using OPENQUERY Function to Execute Queries
The OPENQUERY function allows you to execute a pass-through query on the remote data source using an SQL query written in the remote data source’s syntax. Here is an example of how to use the OPENQUERY function to query data from an Oracle database using Linked Server:
| SELECT * | FROM OPENQUERY(LinkedServerName,’SELECT * FROM SCOTT.CUSTOMERS’); |
In this example, we are executing an SQL statement on the remote Oracle database using the ‘SELECT * FROM SCOTT.CUSTOMERS’ query, which will return all the rows from the ‘CUSTOMERS’ table owned by the ‘SCOTT’ user.
Joining Tables Across Multiple Data Sources
One of the most powerful features of Linked Servers in SQL Server is the ability to join tables across multiple data sources, including SQL Server instances, Oracle, MySQL, and other data sources.
Here is an example of how to join SQL Server and Oracle tables using Linked Server:
| SELECT * | FROM LinkedServerName..SCOTT.CUSTOMERS C | INNER JOIN dbo.ORDERS O | ON C.CUSTOMERID = O.CUSTOMERID; |
In this example, we are joining the ‘CUSTOMERS’ table in the Oracle database to the ‘ORDERS’ table in the local SQL Server database using the Linked Server’s name and four-part naming convention. By joining the data from two separate data sources, we can create a more comprehensive view of the data across different departments and organizations.
Linked Server Management
When you create a Linked Server, you can perform basic operations, such as viewing and modifying the Linked Server, dropping the Linked Server, and testing the connection. Let’s explore how to manage Linked Servers in SQL Server.
Viewing and Modifying Linked Servers
To view and modify the Linked Servers, you can use the ‘sp_linkedserver’ system stored procedure. Here is an example of how to view the Linked Server’s properties:
| EXEC sp_linkedserver | ‘LinkedServerName’; |
This will return information about the Linked Server, including the Linked Server name, product name, data source, provider, security context, and location. You can modify the Linked Server’s properties by using sp_addlinkedserver and sp_dropserver stored procedures.
Dropping a Linked Server
To drop a Linked Server, you can use the sp_dropserver system-stored procedure. Here is an example of how to drop a Linked Server:
| EXEC sp_dropserver | ‘LinkedServerName’, ‘droplogins’; |
This will remove the Linked Server from the SQL Server instance, including all the metadata associated with it. You can also drop the remote logins by setting the second parameter to ‘droplogins’.
Testing the Connection to a Linked Server
To test the connection to a Linked Server, you can use the sp_testlinkedserver system-stored procedure. Here is an example of how to test the connection to a Linked Server:
| EXEC sp_testlinkedserver | ‘LinkedServerName’; |
This will test the connection to the Linked Server and return a result set with information about the Linked Server and its version, provider name, and provider type.
FAQ
What are the benefits of using Linked Servers?
The main benefit of using Linked Servers is that they allow you to connect and manage data from multiple data sources, including other SQL Server instances, Oracle, MySQL, and more, using T-SQL, which is the native language of SQL Server. Linked Servers provide a way to treat all data sources as one by registering and providing a name for each data source, which can then be used to access and query data from the same SQL Server instance or even remotely via a network.
What is the four-part naming convention?
The four-part naming convention is a way of referencing objects that reside in a different database or server. The first part of the naming convention is the Linked Server’s name, followed by the database’s name or schema name, then the object’s name, and finally, the owner’s name. For example, ‘LinkedServerName..SCOTT.CUSTOMERS’ refers to the ‘CUSTOMERS’ table owned by the ‘SCOTT’ user.
What are the security concerns of using Linked Servers?
Linked Servers can introduce security risks because they allow remote access to data across different data sources. When creating a Linked Server, you need to ensure that the appropriate security measures are in place to prevent unauthorized access to the data sources. You should also ensure that the connections to the Linked Server are encrypted and that the credentials are safely stored and encrypted.
Can I join tables across multiple data sources using Linked Servers?
Yes, one of the most powerful features of Linked Servers in SQL Server is the ability to join tables across multiple data sources, including SQL Server instances, Oracle, MySQL, and other data sources. By joining the data from two separate data sources, you can create a more comprehensive view of the data across different departments and organizations.
What are the limitations of using Linked Servers?
There are a few limitations to using Linked Servers. First, you need to ensure that the server that hosts the Linked Server has the appropriate hardware and software resources to handle the remote connection requests. Second, the performance of the queries that involve multiple data sources can be slower than those involving only one data source. Third, you need to ensure that the data types and collation sequences are compatible across all data sources.