How do you calculate a running total in SQL?

In SQL you can compute a running total with a windowed SUM. For example:

SELECT id,
       amount,
       SUM(amount) OVER (ORDER BY id
                         ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS running_total
FROM sales;

This calculates the cumulative sum up to each row. In Spark or Pandas you can use the same window syntax or the cumulative sum function. The operation is linear in the number of rows and requires only a single pass over the data.