你的方法很好。您只需要一个相关的子查询：

update table_yy yy
    set sum_of_x_and_y = (select sum(xx.row_x) + sum(xx.row_y)
                          from table_xx xx
                          where xx.class_id = yy.class_id
                         );

在许多情况下，这将有更好的性能，特别是如果你有一个索引 table_xx(class_id) .

这里有一个查询应该可以完成这项工作

UPDATE table_yy, (
    SELECT class_id, SUM(table_xx.row_x + table_xx.row_y) AS sum_of_x_and_y
    FROM table_xx
    GROUP BY table_xx.class_id 
) AS table_sum
SET table_yy.sum_of_x_and_y = table_sum.sum_of_x_and_y
WHERE table_yy.class_id = table_sum.class_id