Query debugging

Study the pattern

Debug SQL in layers: joins, filters, grouping, and sorting. Predict the output before running the example.

SELECT c.name,SUM(o.amount) AS revenue FROM customers c JOIN orders o ON o.customer_id=c.customer_id WHERE o.amount>0 GROUP BY c.customer_id,c.name HAVING SUM(o.amount)>2000 ORDER BY revenue DESC;

Explain it from memory

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

WHERE filters rows before grouping; HAVING filters groups after.

Run SQL in your browser

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

WHERE filters rows before grouping; HAVING filters groups after.

Consolidate the skill

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