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.