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http://www.sommarskog.se/dynamic_sql.html Get Data from another Database
If you for some reason have your application spread over two databases, what you absolutely not should do is to have code that says:

SELECT ... FROM otherdb.dbo.tbl JOIN ...
This is bad, because if someone asks for a second environment on the same server, you have a lot of code to change.

The best solution for this particular problem is to use synonyms, added in SQL 2005:

CREATE SYNONYM otherdbtbl FOR otherdb.dbo.tbl
You can then refer to otherdb.dbo.tbl as just otherdbtbl. If there is a need for a second set of databases, you only have to update the synonyms, and there is no need to use dynamic SQL.

Yet a way to avoid dynamic SQL is to use stored procedures for all inter-database communication. That is, if you are in db1 and need to get data from db2, you call a stored procedure in db2. This can be dynamic, because EXEC permits you to specify a variable that holds the name of the procedure to execute.

SELECT @dbname = quotename(dbname) FROM ...
SELECT @sp = @dbname + '..some_sp'
EXEC @ret = @sp @par1, @par2...
If you want to get result sets back from db2, look at my article How to Share Data between Stored Procedures for suggestions.

There may still be cases you may find that dynamic SQL is the only feasible situation. This can be done in two ways. The most obvious is:

SELECT @dbname = quotename(dbname) FROM ...
SELECT @sql = ' SELECT ... FROM ' + @dbname + ' .dbo.otherdbtbl ' +
' JOIN dbo.localtbl ... '
EXEC sp_executesql @sql, @params, ...
But, if the query is complex, and most of the tables are in the remote database you can also do:

SELECT @sql = ' SELECT ... FROM dbo.othertbl ' +
' JOIN ' + quotename(db_name()) + '.dbo.localtbl ... '
SELECT @dbname = quotename(dbname) FROM ...
SELECT @sp_executesql = @dbname + '..sp_executesql'
EXEC @sp_executesql @sql, @params, ...
As above, I make use of that you can specify the procedure name dynamically with EXEC. The trick here is that when you specify a system stored procedure in three-part notation with the database name, the procedure executes in the context of that database. Thus, the dynamic SQL in this example runs in @dbname, not the current database.