Syntax:
SUM([DISTINCT] expr) [over_clause]

Returns the sum of expr. If the return set has no rows, SUM() returns
NULL. The DISTINCT keyword can be used to sum only the distinct values
of expr.

If there are no matching rows, SUM() returns NULL.

This function executes as a window function if over_clause is present.
over_clause is as described in
https://dev.mysql.com/doc/refman/8.0/en/window-functions-usage.html; it
cannot be used with DISTINCT.

URL: https://dev.mysql.com/doc/refman/8.0/en/group-by-functions.html

