SQL server or MySQL ?
For SQL Server, use full qualified object names.
[DbName].[schema name (default 'dbo')]. [TableName].[Column name]
Ex.
Select * from
DbA.dbo.Table1 A
inner join
DbB.dbo.Table2 B
on A.Col1 = B.Col1
where A.Col2 >0
You can even join two databases on two...