joining tables from multiple data sources without database links and without temp tables

Foreign Tables via Foreign Data Wrappers from SQL/MED. SQL/MED is part of the SQL Standard. These wrappers provide access to external data sources.

SQL/MED on wikipedia

and without temp tables

What an odd and arbitrary restriction. Why should that apply?

sorry, on an initial thought, it sounded like setting up yet another database for temporary tables

thanks to other monks in this thread, I was able to get an overview of temp tables and it is not anyway a standing up another instance

so yes, I will work on temp tables. obviously, you monks -- @hippo and others are more experienced, if you have any other advice, I will greatly appreciate it...

What an odd and arbitrary restriction.

I'm beginning to think hippo, that this is either a wind up or the data is really coming from somewhere other than an RDBMS such as a flat or structured text file.

so how would one go about joining tables from multiple data sources without database links

One wouldn't!

If you have multiple data sources which are in different databases, then you need to link to them somehow. Data cannot flow from one place to another without some form of link...

and without temp tables...

Why would you want to???
The creators of RDBMSs and languages give us the facilities to solve problems. It is up to us to use them in a sensible way. Choosing not to use one or more is a bit like trying to cross a river by paddling when a perfectly good boat is ready and available.

I know of no modern RDBMS that does not support temporary tables.