Your code is missing GROUP BY and will throw an error without it.
Tableb places no restriction on which rows of Tablea are selected for output because it's left-joined. As Jeff pointed out, it's irrelevant to your query.
What are you really trying to do here? It's highly unlikely that this query is returning the results you are expecting.
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden