Have you ever wondered how SQL Server allows you to stitch multiple databases together and run queries that span multiple database servers? The answer lies in the powerful feature called Linked Servers.

Linked Servers in SQL Server enable you to establish connections to other database servers, not just SQL Servers, and perform distributed queries. This means you can access and manipulate data from multiple servers as if they were part of a single database.

To create a linked server, you can use the sp_addlinkedserver stored procedure. Here’s an example:

EXEC master.dbo.sp_addlinkedserver
@server = N'MyLinkedServer',
@srvproduct = N'',
@provider = N'SQLNCLI',
@datasrc = N'192.168.1.69,1432', -- IP address and port number of the destination server
@catalog = N'AcctDb'; -- database name

Once the linked server is created, you can run queries that involve tables from both the local server and the linked server. For example:

SELECT cust.name, acct.balance
FROM custdb.customers cust, MyLinkedServer.AcctDb.dbo.Accounts acct
WHERE acct.custid = cust.id

Behind the scenes, when you execute a query involving a linked server, SQL Server performs a series of steps to establish the connection and retrieve the data. Let’s take a closer look at what happens:

1. The main SQL Server instance opens a connection to the linked server and authenticates.
2. The main server retrieves information about the linked server, such as session ID, collation, and schema information for the tables involved in the query.
3. The main server prepares the query plan by calling internal stored procedures and obtaining statistics about the tables.
4. The main server formulates the query to be executed on the linked server, based on the nature of the query and the statistics.
5. The main server executes the formulated query on the linked server and retrieves the result set.

All of this happens transparently to the client, making it easy to work with linked servers in SQL Server.

It’s important to note that linked servers can be used not only with SQL Server instances but also with other types of databases, such as Oracle, DB2, PostgreSQL, and even non-relational databases like CSV files and spreadsheet files. However, the ability to run distributed transactions may vary depending on the type of database.

While linked servers provide a powerful way to access data from multiple servers, it’s essential to be aware of potential issues that may arise. For example, schema locks that are not properly released can lead to trouble. Additionally, you may encounter unexpected queries sent by other servers as linked servers.

In conclusion, understanding linked servers in SQL Server gives you the ability to leverage the full potential of distributed queries and access data from multiple servers seamlessly. It’s a testament to the excellent work done by the Microsoft team in making this complex process transparent to users.

So, the next time you need to work with data from multiple servers, consider using linked servers in SQL Server to simplify your tasks and improve efficiency.