How to Join Tables Across Different Databases in SQL Server?

Joining Tables from Different Databases in SQL Server

In SQL Server, it is possible to join tables from different databases as long as they reside on the same server. This capability enables database developers and analysts to link data across disparate schemas.

Syntax

The syntax for joining tables from different databases is similar to that of joining tables within the same database. However, the table names must be fully qualified, specifying both the database name and the schema (if applicable):

SELECT *
FROM Db1.Schema1.Table1 AS t1
JOIN Db2.Schema2.Table2 AS t2
ON t1.ColumnA = t2.ColumnB

Example

Suppose you have two databases, 'Db1' and 'Db2', on the same server. 'Db1' contains a table named 'Clients' with a column 'ClientId', and 'Db2' contains a table named 'Messages' with a column 'ClientId'. These tables share a relationship through the 'ClientId' column.

To perform a join between these tables, you would use the following query:

SELECT *
FROM Db1.dbo.Clients AS c
JOIN Db2.dbo.Messages AS m
ON c.ClientId = m.ClientId

This query will retrieve all rows from the 'Clients' and 'Messages' tables where the 'ClientId' column matches in both tables. The 'dbo' schema is assumed in this example, but you should specify the appropriate schema if your tables are located in different schemas.

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