>>So, turn the UPDATE into select into CTE first and then UPDATE CTE if there is always 1 to 1 relation between table to update and the other tables.
>
>If this would be a single update, that would do. I would be able to select into a temporary table and use that table in the second command (the update). But, in this case, this is a massive update of all the records in the table so I do not think this approach could work.

Why would not you post the actual UPDATE command (you can disguise column names if you want) and then it will be easier to give you the code. In any case, I don't see why the suggested approach is not going to work. You can always split one command into 2 using CTE (common table expression).