Lesson 8 · Expressions and a defensible denominator

Use a defensible average

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AVG skips NULL values; it does not treat a missing finish time as zero. An average needs both a stated population and a stated unit.

ROUND(AVG(finish_minutes), 2) rounds the final average. Averaging individually rounded values is a different computation.

Worked example

SELECT ROUND(AVG(finish_minutes), 2) AS avg_minutes FROM results WHERE race_type = 'Marathon';

Your task

For Half Marathon results, return COUNT(finish_minutes) AS finishers, ROUND(AVG(finish_minutes), 2) AS avg_minutes, MIN(finish_minutes) AS fastest and MAX(finish_minutes) AS slowest.

Predict or plan before trying

Give me a hint

AVG, MIN and MAX ignore NULLs. Use COUNT(finish_minutes) for the number that contributes to the average.

Compare with a worked answer

SELECT COUNT(finish_minutes) AS finishers, ROUND(AVG(finish_minutes), 2) AS avg_minutes, MIN(finish_minutes) AS fastest, MAX(finish_minutes) AS slowest FROM results WHERE race_type = 'Half Marathon';

Using an answer is useful practice. Record what you learned; it does not demonstrate independent mastery.

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Query results

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Explain your evidence

Why does replacing missing finish times with zero change the business answer? What population and units must accompany this average?

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