This is an excellent question, I thoroughly enjoyed it. I guess the reason why initially it is tempting to select 0 as a correct answer is incorrect assumption about when does the DSql gets injected. In other words, if the insert into portion was a part of the DSql itself then the answer would be a no-brainer (0). To figure that the execution works as it is explained is not easy, but of course makes perfect sense. There is an insert into followed by some statement to parse, and the engine expects the latter to possibly have something selected from somewhere. The select portion of the DSql does it, the delete does not change the originally selected data as it is already on the heap, and so the insert of 4 rows takes place.

I wish we had more questions like this! Thank you!

Oleg