Query with multiple databases

Hi，@Jonathan Brotto
The key point is that whether the two databases are on the same server.
1.Two databases on the same server
If you have access to both databases, and your table keys match, you can join across databases by specifying the fully qualified name :[Database].[Schema].[Table]
for example,

SELECT T1.ID, T1.name,T2.ID,T2.name  
FROM DatabaseA.dbo.Table1 T1  
JOIN DatabaseB.dbo.Table2 T2   
on T2.ID = T1.ID  

2.Two databases on different servers :

exec sp_addlinkedserver 'alias','','sqloledb','ip address'  
exec sp_addlinkedsvlogin 'alias','false',null,'uesername','password'  
go  
SELECT ...  
FROM alias.DatabaseA.dbo.Table1 T1  
JOIN alias.DatabaseB.dbo.Table2 T2   
on ...  

Note:Two servers can access the database only if they are allowed to communicate

Best regards,
LiHong

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