Lesson 13 · Filter rows, then filter groups

Filter rows before groups, groups after aggregation

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Read the idea

WHERE removes input rows before aggregation. HAVING removes groups after aggregation. An average over all menu items is different from an average over only expensive items.

A good check reconciles the grouped counts with a total computed on the same filtered population. Matching a number is evidence only when both queries measure the same thing.

Worked example

SELECT category, COUNT(*) AS items FROM menu_items WHERE price >= 5 GROUP BY category HAVING COUNT(*) >= 2 ORDER BY category;

WHERE price >= 5 changes the input population first. GROUP BY collects those expensive items into categories, and HAVING keeps only categories with at least two remaining items.

Moving the count condition into WHERE is not equivalent. Removing WHERE is valid SQL but changes both the category counts and the business question.

Your task

Return category and COUNT(*) AS item_count for categories with at least three menu items. Include all price levels; order by category.

Predict or plan before trying

Give me a hint

The threshold applies to the number of rows in a group, so it belongs in HAVING.

Compare with a worked answer

SELECT category, COUNT(*) AS item_count FROM menu_items GROUP BY category HAVING COUNT(*) >= 3 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

Why would WHERE price >= 3 answer a different question?

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