How to select data of a table from another database in SQL Server?

You need sp_addlinkedserver()

Example:

exec sp_addlinkedserver @server = 'test'

then

select * from [server].[database].[schema].[table]

In your example:

select * from [test].[testdb].[dbo].[table]

sql sql-server database linked-server

In SQL Server 2012 and above, you don't need to create a link. You can execute directly

SELECT * FROM [TARGET_DATABASE].dbo.[TABLE] AS _TARGET

I don't know whether previous versions of SQL Server work as well

sql sql-server database linked-server

I've used this before to setup a query against another server and db via linked server:

EXEC sp_addlinkedserver @server='PWA_ProjectServer', @srvproduct='',@provider='SQLOLEDB', @datasrc='SERVERNAME\PWA_ProjectServer'

per the comment above:

select * from [server].[database].[schema].[table]

e.g.

select top 6 * from [PWA_ProjectServer].[PWA_ProjectServer_Reporting].[dbo].[MSP_AdminStatus]

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