Data quality checks

Study the pattern

Counts, ranges, and NULL checks help profile data. Predict the output before running the example.

SELECT COUNT(*) AS rows_total,COUNT(country) AS countries_filled,MIN(customer_id) AS min_id,MAX(customer_id) AS max_id FROM customers;

Explain it from memory

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

COUNT(column) ignores NULL; COUNT(*) counts all rows.

Run SQL in your browser

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

COUNT(column) ignores NULL; COUNT(*) counts all rows.

Consolidate the skill

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