Lesson 7 · Whole-table aggregates

A one-row dashboard

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Read the idea

An aggregate summarizes the rows that reach it. COUNT(*) counts rows; COUNT(column) counts non-NULL values. SUM, AVG, MIN and MAX summarize numeric values.

Apply WHERE before the aggregate to define the population. ROUND changes the precision of a calculated value, not the underlying records.

Worked example

SELECT COUNT(*) AS runner_count FROM runners;

One input row represents a runner. The output is one summary row.

Your task

Show COUNT(*) AS runner_count, MIN(id) AS first_runner and MAX(id) AS last_runner from runners.

Predict or plan before trying

Give me a hint

Put all three aggregates in the same SELECT list.

Compare with a worked answer

SELECT COUNT(*) AS runner_count, MIN(id) AS first_runner, MAX(id) AS last_runner FROM runners;

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

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Query results

Run your query to see the result here.

Explain your evidence

Why does this result have one row rather than one row per runner?

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