How or where can I do a select comparing two tables from different databases?
Example:
ventas_cabecera in both Which of the platforms gives me an option to consult data from an external DB?
You could create a linked server from Sql Server to reference the db of Oracle
Connecting SQL Server and Oracle Using Linked Servers
Creating a Linked Server for Oracle in 64bit SQL Server
You can link it using the visual tools or using sp_addlinkedserver
Once the server is linked, you can use it in the queries that you make by joining in a JOIN the tables of two different databases.
To refer to the likeada table, you should only respect the nomenclature
nombre_linked_server.nombre_catalog.nombre_schema.nombre_tabla
In SQL Server you have a functionality called " Linked servers "that allows access to databases of other engines such as Oracle or MySQL (I use it with the latter to obtain data from MantisBT )
In this link to the TechNet have information on how create a linked server
Once you have created and configured the linked server, you can launch queries using openquery considering the query that you throw to the linked server as if it were a table more and making queries similar to the following:
select qMP.name as Project, SUM(qW.WorkTime)
from openquery(PBUGS, 'select id, name from mantis_project_table') as qMP
left outer join work_reporting as qW ON qW.MantProject = qMP.name
group by qMP.name