Running Total with a Window Function

-- Running total of order values, oldest first.
-- The explicit ROWS frame matters: the default frame (RANGE) gives tied dates the same total
-- and is slower in SQL Server. The id in ORDER BY breaks ties between orders on the same date.
SELECT id,
       order_date,
       total,
       SUM(total) OVER (
           ORDER BY order_date, id
           ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
       ) AS running_total
FROM orders;