Lesson 10 · Numbers and missing values

Repair integer division before rounding

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When both operands are integers, SQLite division truncates toward zero. ROUND(7 / 2, 1) rounds an already truncated 3; it cannot recover the missing fraction. Put a real-valued operand into the division first, such as 7 * 1.0 / 2.

A rate needs a named numerator and denominator. A mean number of scans per package is different from the percentage of packages delivered. Neither a decimal place nor an attractive alias establishes the business meaning.

NULLIF(denominator, 0) changes a zero denominator into NULL. This leaves an undefined ratio missing rather than labeling it zero. Explain why the denominator is valid before interpreting the result.

Worked example

SELECT 7 / 2 AS integer_result, ROUND(7 * 1.0 / 2, 1) AS real_result, 7.0 / NULLIF(0, 0) AS undefined_ratio;

The first expression demonstrates truncation; the second preserves the fractional result. A missing ratio communicates an undefined operation, not a successful zero rate.

The fixed denominator is justified only by this snapshot. Recheck SELECT COUNT(*) FROM packages before reusing the calculation with new data.

Your task

The schema shows 40 packages. Return COUNT(*) AS scan_count and ROUND(COUNT(*) * 1.0 / 40, 1) AS mean_scans_per_package from scans.

Predict or plan before trying

Give me a hint

Make the numerator real before dividing. Round only the completed ratio.

Compare with a worked answer

SELECT COUNT(*) AS scan_count, ROUND(COUNT(*) * 1.0 / 40, 1) AS mean_scans_per_package FROM scans;

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 is this mean not the percentage delivered? What would have to change if another package were added?

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