SQL joins: why your row count changed
SELECT chooses columns, FROM chooses a table, and WHERE filters rows. Practice: SELECT name, revenue FROM sales WHERE revenue > 100; Explain which rows remain.
SQL joins: why your row count changed
Select and filter
SELECT chooses columns, FROM chooses a table, and WHERE filters rows. Practice: SELECT name, revenue FROM sales WHERE revenue > 100; Explain which rows remain.
Join related tables
JOIN combines records using a relationship. INNER JOIN keeps matching rows; LEFT JOIN keeps every left row. Practice: join customers to orders on customer_id and inspect customers without orders.
Group and aggregate
GROUP BY forms groups; COUNT counts rows and SUM totals values. WHERE runs before grouping; HAVING filters groups. Practice: total revenue by region and keep totals above 1000.
Your next step
Try the exercises with a small dataset. Write down your assumptions, check the results against the source, and explain one thing you would improve.