How do I calculate a running SUM on a SQLite query?

Question

How do I get a column that is the sum of all the values before of another column?

Answer 1

As of SQLite 3.25.0, since 2018-09-15, window functions and their keyword OVER are supported. The answer to your question is now easy:

SELECT Country, Gdp, SUM(Gdp) OVER (ROWS UNBOUNDED PRECEDING)
FROM CountryGdp;

This is the minimal query that does what you request, but it doesn't define any ordering so here is a more proper way of doing it.

SELECT
    Country,
    Gdp,
    SUM(Gdp) OVER (
        ORDER BY Country -- Window ordering (not necessarily the same as result ordering!)
        ROWS BETWEEN -- Window for the SUM includes these rows:
            UNBOUNDED PRECEDING -- all rows before current one in window ordering
            AND CURRENT ROW -- up to and including current row.
        ) AS RunningTotal
FROM CountryGdp
ORDER BY Country;

In any way, the query should run in O(N) time.

Answer 2

You can do it by joining the table with itself (performing a so-called Cartesian or cross join). See the following example.

SELECT a.name, a.gdppc, SUM(b.gdppc)
FROM gdppc AS a, gdppc AS b WHERE b.gdppc <= a.gdppc 
GROUP BY b.id ORDER BY a.gdppc;

Given a table containing countries and their per capita GDP it will give you a running total of the GDP figure.

Democratic Republic of Congo|329.645|329.645
Zimbabwe|370.465|700.11
Liberia|385.417|1085.527
Burundi|399.657|1485.184
Eritrea|678.954|2164.138
Niger|711.877|2876.015
Central African Republic|743.945|3619.96
Sierra Leone|781.594|4401.554
Togo|833.803|5235.357
Malawi|867.063|6102.42
Mozambique|932.511|7034.931
...

Note that this can be a very resource-intensive operation, because if a table has N elements it will create a temporary table with N*N elements. I would not perform it on a large table.