SQL Aggregate Calculations

What & Why

Once an aggregate function produces a value, it behaves exactly like any other number in the query — it can be added, multiplied, divided, or combined with other aggregates using ordinary arithmetic operators, right there in the same SELECT list. This is how a single query ends up computing a genuinely derived business metric, not just a raw sum or average.

See How It Works

SELECT
  source,
  COUNT(*) AS leads,
  COUNT(*) FILTER (WHERE converted_at IS NOT NULL) AS conversions,
  ROUND(100.0 * COUNT(*) FILTER (WHERE converted_at IS NOT NULL) / NULLIF(COUNT(*), 0), 2) AS conversion_rate
FROM marketing.leads
GROUP BY source
ORDER BY conversion_rate DESC NULLS LAST, source;