通过MySQL聚合查询执行更新

I am attempting to update a field on a single table with the results of a sum statement from another table. In order to get the results I have to tie together two tables and sum a qty field in one of those two tables. I then need to set the summed qty to a field in a third table. I've looked around for a solution, and have found some answers, but they weren't super detailed answers and I have been unable to make it work for my situation.I am attempting to update a field on a single t