Why does this SQL SUM return NULL for empty?

Why does this SQL SUM return NULL for empty result. The SQL code sums a column without COALESCE. In SQL SUM of zero rows returns NULL not zero. In SQL APIs this breaks calculations and causes NaN downstream.

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https://www.youtube.com/watch?v=gG4KtBljZ-o

Why does this SQL SUM return NULL for empty? #sqlreliability

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