In case you want to use SQL code already written, take a look at this: victoriayudin.com/.../sql-view-for-current-receivables-aging-detail-in-dynamics-gp.
Also, a little caution about your code - that will ONLY work if the document number starts with 'PYMNT'. It will not capture returns or credit memos and if in the future you change the numbering for your payments it might break, as well. (For example, our payments start with 'PMT'.) So while it may work for the subset of data you are looking at now, it may not be 'generic' enough to work consistently in the future.