Demystifying Linked Server in SQL Server for Devs

Greetings, Dev! If you are looking to connect multiple SQL Server instances or working with data from multiple databases in different locations, then understanding the concept of linked server in SQL Server is imperative. This article aims to provide you with a comprehensive guide on linked servers, from the basics to the advanced techniques, and to equip you with the ability to use this feature effectively in your projects.

Table of Contents

• Introduction to Linked Server
• Configuring Linked Server
• Types of Linked Server
• Using Linked Server in Queries
• Securing Linked Server
• Improving Performance with Linked Server
• Troubleshooting Linked Server
• FAQs about Linked Server

Introduction to Linked Server

Linked Server is a database object in SQL Server that allows you to connect to remote data sources, including other SQL Server instances, Oracle, MySQL, Access, and many others, and access data from those sources as if they were part of your local database. This eliminates the need for complex ETL processes or stored procedures to move data between databases, and allows you to query and join data across different platforms with ease.

Linked Server is a powerful feature, but it needs to be used with caution. If not configured properly, it can cause performance issues, security vulnerabilities, and other troubles. In the following sections, we will discuss in detail how to configure, use, and troubleshoot Linked Server in SQL Server.

Configuring Linked Server

Step 1: Enable Distributed Query

Before you can create a linked server, you need to ensure that the Distributed Query feature is enabled in SQL Server, which allows you to query data from remote sources. To enable Distributed Query, execute the following T-SQL statement:

sp_configure 'show advanced options', 1;GORECONFIGURE;GOsp_configure 'Ad Hoc Distributed Queries', 1;GORECONFIGURE;GO

You also need to ensure that the SQL Server Browser service is running, which allows you to connect to the remote server by name instead of IP address.

Step 2: Create Linked Server

To create a linked server, you can use the SQL Server Management Studio wizard or execute the following T-SQL statement:

EXEC sp_addlinkedserver@server = 'MyLinkedServer',@srvproduct = ' ',@provider = 'SQLNCLI',@datasrc = 'MyRemoteServer';

This statement creates a linked server named ‘MyLinkedServer’ that uses the SQLNCLI (SQL Native Client) provider to connect to the remote server named ‘MyRemoteServer’. You can replace the provider with another one that supports your remote database.

Step 3: Configure Linked Server Properties

After creating a linked server, you can configure its properties, such as security, connection timeout, and data access options. To do this, right-click on the linked server in Object Explorer and select ‘Properties’.

Step 4: Test Linked Server Connection

Before using a linked server in your queries, you should test its connection to the remote server. To do this, execute the following T-SQL statement:

EXEC sp_testlinkedserver 'MyLinkedServer';

This statement should return a message indicating whether the connection was successful or not.

Types of Linked Server

SQL Server Linked Server

The most common type of linked server is the SQL Server Linked Server, which allows you to connect to other SQL Server instances and access their data. You can use this type of linked server to join data from different SQL Server databases or instances, or to migrate data from one server to another.

Provider Linked Server

A Provider Linked Server is a linked server that uses a non-SQL Server provider to connect to a remote data source. For example, you can use a Provider Linked Server to connect to an Oracle database or an Excel spreadsheet. You need to install the provider on the SQL Server machine before creating a Provider Linked Server.

Custom Linked Server

A Custom Linked Server is a linked server that is created by implementing the IDBInitialize interface in a COM object. This type of linked server is rarely used.

Using Linked Server in Queries

Basic Linked Server Query

To query data from a remote server using a linked server, you need to prefix the table name or view name with the linked server name and database name. For example, if you have a linked server named ‘MyLinkedServer’ that connects to a database named ‘MyRemoteDB’, and you want to query a table named ‘Customers’ in that database, you can use the following syntax:

SELECT * FROM MyLinkedServer.MyRemoteDB.dbo.Customers;

This statement retrieves all data from the ‘Customers’ table in the ‘MyRemoteDB’ database through the ‘MyLinkedServer’ linked server.

Joining Tables from Different Servers

You can also use a linked server to join tables from different servers in a query. For example, if you have two linked servers named ‘MyLinkedServer1’ and ‘MyLinkedServer2’, and you want to join a table named ‘Orders’ in the ‘MyRemoteDB1’ database on ‘MyLinkedServer1’ with a table named ‘Customers’ in the ‘MyRemoteDB2’ database on ‘MyLinkedServer2’, you can use the following syntax:

SELECT * FROMMyLinkedServer1.MyRemoteDB1.dbo.Orders OINNER JOIN MyLinkedServer2.MyRemoteDB2.dbo.Customers CON O.CustomerID = C.CustomerID;

This statement joins the ‘Orders’ table on ‘MyRemoteDB1’ with the ‘Customers’ table on ‘MyRemoteDB2’ using the ‘CustomerID’ column as the join condition.

Using OPENQUERY

The OPENQUERY function allows you to execute a query on a remote server and retrieve the results as if the query was executed locally. For example, if you want to retrieve the top 10 orders from a table named ‘Orders’ in a remote database, you can use the following syntax:

SELECT * FROM OPENQUERY(MyLinkedServer,'SELECT TOP 10 * FROM MyRemoteDB.dbo.Orders ORDER BY OrderDate DESC');

This statement executes the inner query on the remote server and retrieves the top 10 orders, and then returns the results to the local server as if they were part of a local table.

Securing Linked Server

Configuring Linked Server Security

Linked Server can pose security risks, especially if the remote server is not secure or trusted. Therefore, you should configure the security settings of Linked Server carefully to prevent unauthorized access or data leakage. Some security options you can configure for Linked Server are:

• Mapping a local user to a remote user for authentication using the sp_addlinkedsrvlogin stored procedure. This ensures that only authorized users can access the linked server.
• Using a remote login or a Windows domain account for authentication, instead of a SQL Server login, for better security.
• Configuring the RPC and RPC Out options to limit the types of remote procedure calls that are allowed from the linked server, and to prevent the linked server from executing remote commands or inserting data into the local server.

Improving Performance with Linked Server

Using Query Hints

When you use Linked Server in a query, you should optimize the query to ensure the best performance. One way to do this is to use query hints to tell SQL Server how to execute the query. Some query hints you can use for Linked Server are:

• TOP: If you only need to retrieve a small number of rows from the remote server, you can use the TOP query hint to limit the number of rows returned. This can reduce the amount of data transferred over the network and improve query performance.
• FORCE ORDER: If you want to control the order of table joins or subqueries in the query, you can use the FORCE ORDER query hint to instruct SQL Server to follow your order.
• NOLOCK: If you are querying a read-only table on the remote server and you don’t care about dirty reads or phantom rows, you can use the NOLOCK query hint to avoid locking the table and improve query performance.

Configuring Linked Server Options

You can also improve the performance of Linked Server by configuring its options to optimize data access and network communication. Some options you can configure for Linked Server are:

• Collation Compatible: If the collation of the local and remote servers is different, you can set the Collation Compatible option to false to avoid unnecessary collation conversions, which can improve performance.
• Lazy Schema Validation: If the remote server has a large number of tables or views, you can set the Lazy Schema Validation option to true to delay the validation of schema until the first query on a specific object. This can reduce the startup time of Linked Server.
• Enable Promotion of Distributed Transactions: If you need to execute a distributed transaction that involves the local and remote servers, you can set the Enable Promotion of Distributed Transactions option to true to allow SQL Server to promote the transaction to a distributed transaction, which can improve transactional consistency.

Troubleshooting Linked Server

Common Issues and Solutions

Linked Server can be a complex feature, and you may encounter various issues when using it. Here are some common issues and their solutions:

FAQs about Linked Server

What is the purpose of Linked Server?

The purpose of Linked Server is to allow you to connect to and access data from remote data sources, including other SQL Server instances, Oracle, MySQL, Access, and many others, and to query and join data across different platforms with ease.

How do I create a linked server?

You can create a linked server using the SQL Server Management Studio wizard or by executing the sp_addlinkedserver stored procedure. Ensure that the Distributed Query feature is enabled and that the SQL Server Browser service is running before creating a linked server.

How do I use a linked server in a query?

To use a linked server in a query, you need to prefix the table name or view name with the linked server name and database name. You can also use the OPENQUERY function to execute a query on a remote server and retrieve the results as if the query was executed locally.

How do I secure a linked server?

You can secure a linked server by mapping a local user to a remote user for authentication, using a remote login or a Windows domain account for authentication, and configuring the RPC and RPC Out options to limit the types of remote procedure calls that are allowed from the linked server.

How do I troubleshoot a linked server?

You can troubleshoot a linked server by checking the network connection, firewall settings, server name resolution, linked server properties, provider settings, and security configurations. Also, optimize the query using query hints, indexing, and data filtering, and make sure the data types, collation, and schema are compatible between the local and remote servers.

That’s it, Dev! We hope this article has provided you with a thorough understanding of linked server in SQL Server and how to use it effectively in your projects. If you have any questions or feedback, feel free to leave a comment below.