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6Lecture 6: Queries with Aggregates (Pt. 1)Buildings & employees · 3 exercises7Lecture 7: Queries with Aggregates (Pt. 2)Buildings & employees · 3 exercises
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Count rows and known values

Step 1 of 4 · Database: Northshore Coffee

READ & UNDERSTAND

COUNT(*) counts rows. COUNT(calories) counts rows with a non-NULL calorie value. COUNT(DISTINCT category) counts different known categories. Always say what is being counted.

NULL means a value is missing or unknown; it is not automatically zero. AVG ignores NULL values, so an average may describe only part of the menu. State the denominator alongside your conclusion.

Worked example
SELECT COUNT(*) AS items, COUNT(calories) AS known_calories FROM menu_items;

Both aggregates consume the same menu_items rows, but COUNT(calories) ignores rows where that value is NULL. Subtracting known_calories from items reveals how many rows lack that field.

This returns one summary row because there is no GROUP BY. Neither number measures the number of categories; that requires counting distinct category values.

Your question

Return COUNT(*) AS item_count and COUNT(DISTINCT category) AS category_count from menu_items.

Explain your evidence

Why are item_count and category_count answering different questions?

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CURRENT DATABASE

Quick schema · Northshore Coffee

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