SUM

Syntax

SUM(expression)

What it does

SUM() adds values across rows in a group. It is commonly used for totals, revenue, and aggregate reporting.

Example

SELECT
  customer_id,
  SUM(amount) AS lifetime_value
FROM orders
GROUP BY customer_id;

Important behavior

• SUM() ignores NULL values in the input expression
• if all values are NULL, the result is NULL
• use COALESCE when you need a numeric fallback instead of NULL

Common gotchas

• SUM() changes the grain of the result set unless combined with a window function
• grouping logic must be intentional and consistent with the business definition of a segment
• row counts and sums may disagree if filtered rows are excluded before aggregation

Safety checklist

• confirm the intended grouping before aggregating
• decide whether NULL should be treated as zero or excluded
• validate the returned total against expected business numbers before shipping