SQL Server query - 2 databases

Question

I need to make a dynamic query that will span across 2 databases.

Db1: Table1

Db2: Table2

First I will select some items based on hard coded ID's from table1 in db1.

Select * 
from Db1.Table1 
where Id = 123

Table1 has a column called CityId that is part of Db2.Table2 . So when inserting new items in Db2.Table2 , the CityId is an Identity column.

Now I need something like:

use Db1
go

select * 
from Db1.Table1 
where Id = 123

use Db2
go

select * 
from Db2.Table2 
where CityId in (select CityID 
                 from Db1.Table1 
                 where Id = 123)   // how can I solve this cross db query?

Answer 1

If I understand correctly, you just need to use three-part naming:

select t2.*
from Db2..Table2 t2
where t2.CityId in (select t1.CityID from Db1..Table1 t1 where t1.Id = 123) 

The default schema name is usually dbo , so you can be more explicit:

select t2.*
from Db2.dbo.Table2 t2
where t2.CityId in (select t1.CityID from Db1.dbo.Table1 t1 where t1.Id = 123)