How to query across multiple databases - sql

Question

I have a set of data that belongs to two different tables in two different databases. I have performed the query but I am getting a syntax error.
The two databases are:
dbo.Serve and dbo.Back
This is what I am doing:
SELECT Back.[dbo].Back$.Last_refreshed, dbo.Back.Backup_Name, dbo.Back$.Fle, dbo.Back$.Free_Space_TB, dbo.Backup$.Operational_status, dbo.Serve$.Name,
dbo.Back$.Rack_Enclosure, dbo.Back$.Server_Name, dbo.Back$.SIZE_TB, x.field, x.piv, dbo.Serve$.SKU,
CASE
WHEN dbo.Serve$.SKU like 'F1%' or dbo.Serve$.SKU like 'F2%' or dbo.Server_History$.SKU like 'F3%' or
dbo.Serve$.SKU like 'FY14%' or dbo.Serve$.SKU like 'F5%' or dbo.Server_History$.SKU like 'F6%' then 'Group1'
WHEN dbo.Serve$.SKU like 'Gr%' or dbo.Serve$.SKU like 'Fr%' or dbo.Server_History$.SKU like 'Ex%'
or dbo.Serve$.SKU like 'Ga%' or dbo.Serve$.SKU like 'Ho%' or dbo.Serve$.SKU like 'Gen%' then 'Group2'
END AS grp
FROM dbo.Back$
inner join dbo.Serve$ ON dbo.Serve$.Name = dbo.Back$.Server_Name and dbo.Back$.Last_refreshed = dbo.Serve$.Last_refreshed
cross apply (values (dbo.Back$.Free_Space_TB, 'Available'), (dbo.Back$.SIZE_TB - dbo.Back$.Available, 'Used')) AS x(field, piv)
Why would it give me an error on this part?
SELECT Back.[dbo].Back$.Last_refreshed,
as the syntax appears correct
Any help is appreciated.

Answer 1

This Back.[dbo].Back$.Last_refreshed,
does not make sense. You can still use databasename.schemaname.columnname
or a four part name (if you set up linked servers...
instancename.databasename.schemaname.columnname),
but it is not clear what you are referring to with: Back.[dbo].Back$.Last_refreshed