We have said that a NULL value for s.DateOfSale is not in the range we are interested in. This means the rows with NULLs in the s.DateOfSale column (our employees yet to make a sale) will be filtered out. It will also filter out employees with sales in months other than March. We have converted the LEFT JOIN into an INNER JOIN.
James’s fix is to move the filter to the join clause, which eliminates the implicit inner join. When I see a condition like this in a code review, the first question on my mind is whether the correct fix is James’s fix or whether the developer really meant to do an inner join. There’s a potential performance gain from using an inner join over a left outer join (due to being able to drive from either table and thus having a larger number of potential execution plans) if it turns out you really do want to filter all rows and not just making the join criterion more specific.