SQL: Subqueries: Explain what a subquery is and provide an example of how you might use it. Normalization: What is database normalization, and why is it important? Can you give an example of normalization? Transactions: Describe the properties of a transaction in a database. What is ACID, and why is it important? Performance Tuning: How would you optimize a slow-performing SQL query? What tools or techniques would you use? Joins: Explain the differences between INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN.