How Can I Efficiently Join Data from Separate Databases?

Combining Data from Separate Database Systems

Working with data spread across multiple, independent databases often requires joining tables for complete data analysis. Here are two effective methods:

1. Establishing Database Links:

Create a database link using the appropriate stored procedure (e.g., sp_addlinkedserver in SQL Server) to connect your local database to the remote database. This treats remote tables as if they were local, simplifying query syntax:

<code class="language-sql">-- Querying data from DB1
SELECT *
FROM [MyDatabaseOnDB1].[dbo].[MyTable] tab1
INNER JOIN [DB2].[MyDatabaseOnDB2].[dbo].[MyOtherTable] tab2
ON tab1.ID = tab2.ID</code>

2. Using Remote Query Execution:

The OPENQUERY function lets you execute SQL statements directly on the remote server and return the results to your local database. This can be more performant than database links because the remote server handles query optimization:

<code class="language-sql">-- Retrieve data from the second database server
SELECT *
INTO #myTempTable
FROM OPENQUERY([DB2], 'SELECT * FROM [MyDatabaseOnDB2].[dbo].[MyOtherTable]')

-- Join the temporary table with local data
SELECT * 
FROM [MyDatabaseOnDB1].[dbo].[MyTable] tab1
INNER JOIN #myTempTable tab2 
ON tab1.ID = tab2.ID</code>

Both database links and OPENQUERY provide efficient solutions for joining data across separate databases, enabling more robust and informative queries.

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