Comment by mcdonje

When you add in the ability to reference different tables like that to the piping syntax, it starts to remind me of the M query language: https://docs.microsoft.com/en-us/powerquery-m/quick-tour-of-...

There, each variable can be referenced by downstream steps. Generally, the prior step is referenced. Without table variables, your language implicitly pipes the most recent one. With table references, you can explicitly pipe any prior one. That way, you can reference multiple prior steps for a join step.

I haven't thought through that fully, so there may be gotchas in compiling such an approach down to SQL, but you can already do something similar in SQL CTEs anyway, so it should probably work.