Pipelines - Delete unmatched records

In all likelihood if you tried to identify it in the pipeline and then delete at the same time - you'd have to do the search and deletion one by one inside of a loop which would be taxing on the Pipeline, Quickbase, and be pretty slow. It would likely entail searching ALL your records in Table B - looping each - querying to see if you can find a match in Table A and if the query responds with 0 records in Table A, then delete the record from B. 

That setup though means you're individually querying for every record one by one.

I haven't personally had a use case for it - but you might also look at the Pipeline table/row object. Again - never used it myself - but you could query table A - loop each record and add a row in a Pipeline table with the RID. Then, query for all records in Table B, then loop each record in that and instead of directly querying back to Table A - search the table to see if the associated Table A entry is in your Pipeline table and if it doesn't exist - then delete the Table B record. Same concept as above - but you're using the table like an array that you can index versus querying QB each time. Still slow - but less taxing on QB directly. This would definitely be the better option but comes with more trial and error.