Hi Irfan, it looks like your question fell through the cracks, and we're only now getting to respond to it -- my apologies for that.
It depends what you mean by "mapping values" between databases. Do you mean that you have two tables with essentially the same columns, but in two separate databases, and you want to retrieve data from both? Or do you mean that you need to retrieve data from both tables with a join?
If it's the latter, then you can define a virtual relationship between these two tables, and then create a resource that spans both tables. This is assuming that there is more or less a one-to-one correspondence of (presumably) the primary key, so that you can do a meaningful join (maybe using uid, for instance). That would give you two levels of data, one level for each table.
If you are trying to retrieve data from both tables in one result (say, you have users A-M in database 1 and users N-Z in database 2, and you want these two tables to be treated as one), that can be more challenging. Are these two databases in the same server, or at least same instance? In other words, is the database capable of running a query that spans both tables? If so, you could use a free SQL resource to do a union of both tables.
If that's not the case, say one table is in Oracle and the other is in SQL Server, then it's going to more difficult to query the two tables in unison. Not impossible -- but more difficult. If that's the case, please let us know the details and we'll see if we can be of assistance.