Lesson 12 · One output row per group

Choose the unit of your summary

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Read the idea

GROUP BY creates one result group for each distinct grouping value. Grouping by category answers a category-level question; grouping by category and size creates a finer comparison.

Every selected non-aggregate value should describe the group unambiguously. SQLite can accept a bare column outside GROUP BY, but acceptance does not make an arbitrary item name a meaningful label for a category average.

Worked example

SELECT category, COUNT(*) AS items, ROUND(AVG(price), 2) AS average_price FROM menu_items GROUP BY category ORDER BY category;

Each category becomes one group. COUNT counts its menu rows and AVG summarizes their prices; the result is ordered by category so teams can compare it consistently.

The average is per menu item in this fixture, not weighted by purchases. You cannot describe it as the average price customers paid without appropriate order-item data.

Your task

Return category and COUNT(*) AS item_count for each category, ordered by category.

Predict or plan before trying

Give me a hint

One row should represent one category, not one item.

Compare with a worked answer

SELECT category, COUNT(*) AS item_count FROM menu_items GROUP BY category ORDER BY category;

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

If you add size to GROUP BY, why might the number of result rows increase?

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