Sample Answer : “A subquery is a query nested within another SQL query. It is used to retrieve more complex and specific data or to perform operations that require results from an inner query. For example, in an employee query, you might use a subquery to find employees with above-average salaries.”
2. Explain the difference between INNER JOIN, LEFT JOIN, and RIGHT JOIN. When would you use each?
Description : Assess the candidate’s understanding of different types of JOINs and their application in specific scenarios.
Sample Answer : “INNER JOIN returns rows when spain telegram data there is a match in both tables, LEFT JOIN returns all rows from the left table and matching rows from the right table, and RIGHT JOIN returns all rows from the right table and matching rows from the left table. I use them based on business needs; for example, LEFT JOIN to retrieve all customers with or without orders.”
3. What are indexes in a database and why are they important?
Description : Assesses the candidate’s knowledge of indexes and their impact on query performance.
Sample Answer : “Indexes are structures that improve data retrieval speed by acting as a quick reference to records in a table. They are important because they help optimize SELECT queries, facilitate searching and sorting of data, and improve overall database performance by reducing data access time.”
4. How do you optimize SQL query performance?
Description : Assesses the candidate’s ability to identify and apply query optimization techniques.
Sample Answer : “I optimize query performance using techniques such as creating appropriate indexes, limiting the use of functions in WHERE clauses, avoiding SELECT and selecting only necessary columns, and optimizing subqueries and UNIONs. Additionally, I perform query profiling to identify bottlenecks and tune accordingly.”
5. What are triggers in SQL and when would you use them?
Description : Assesses the candidate’s understanding of triggers and their application in databases.
Sample Answer : “Triggers are stored procedures that are automatically executed in response to certain events in a table, such as INSERT, UPDATE, or DELETE. I use them to apply additional business logic, ensure referential integrity of data, or audit changes to the database.”
