MySQL Coding Challenges: Database Challenges Tutorial

Write a Query to Find Customers with No Purchases

Write a SQL query to list customers who have never made a purchase.

Expected output: A list of customer IDs or names with no corresponding purchase records.

Hint: Use LEFT JOIN and check for NULL in the purchase table.

Optimize a Slow Query

Given a slow query on a large table, identify bottlenecks and rewrite it for better performance.

Expected output: An optimized query with improved execution plan and faster runtime.

Hint: Use EXPLAIN and consider adding indexes or rewriting joins.

To optimize a slow query, analyze it with EXPLAIN to understand its execution plan, add appropriate indexes, avoid SELECT *, filter data early, and consider query restructuring or caching results.

INNER JOIN returns rows with matching keys in both tables, while LEFT JOIN returns all rows from the left table and matched rows from the right table, filling NULLs when no match exists.

Transactions ensure that a group of SQL operations either all succeed or all fail, maintaining data consistency and preventing partial updates that could corrupt data.