KITFORMA · EDITORIAL ANSWER
Why does SUM return NULL for an empty result instead of zero?
KitForma editorial guide. In PostgreSQL, most aggregates other than count return NULL when no input rows contribute. An unknown total and a known zero are not automatically equivalent.
Step-by-step answer
In PostgreSQL, most aggregates other than count return NULL when no input rows contribute. An unknown total and a known zero are not automatically equivalent.
Use
COALESCE(SUM(amount), 0) when the application contract defines an empty total as zero. Test no rows, all-null amounts and an actual zero amount separately. Do not apply COALESCE before understanding missing data: replacing unknown measurements with zero can bias averages. Choose a fallback type compatible with the aggregate, especially for numeric currency or interval results.Sources and verification
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