Percentages and ratios

Study the pattern

Calculate each customer's share of total revenue. Predict the output before running the example.

SELECT customer_id,SUM(amount) AS revenue,ROUND(100.0*SUM(amount)/(SELECT SUM(amount) FROM orders),1) AS revenue_pct FROM orders GROUP BY customer_id ORDER BY revenue DESC;

Explain it from memory

What does today's SQL feature do, and when would you use it?

Use 100.0 to avoid integer division.

Run SQL in your browser

Run the example, then modify one clause and predict how the result changes.

Use 100.0 to avoid integer division.

Consolidate the skill

• Run and inspect the output.
• Change one part and predict the difference.
• Return later and rewrite the query from memory.