Orders AS O GROUP BY O.customer_id )OA ON C.customer_id = OA.customer_id; table while pulling in necessary information from other table(s) referenced in the statement.
If CTEs are your thing, you can even go a step further with this.
If it does, then only one of the join rows will be used to update the target row, but which one will be used is not readily predictable.
Because of this indeterminacy, referencing other tables only within sub-selects is safer, though often harder to read and slower than using a join.
There are two ways to modify a table using information contained in other tables in the database: using sub-selects, or specifying additional tables in the sub-query that produces as many output columns as are listed in the parenthesized column list preceding it.