10 Must Ask SQL Interview Questions For Hiring Managers

Posted on April 17 2024 by Interview Zen Team

Introduction

Structured Query Language (SQL) enables developers to interact with databases, perform queries, update data, and manage database structures efficiently. As data becomes increasingly central to business operations, SQL skills are essential for most software development roles.

According to Stack Overflow’s 2024 Developer Survey, SQL ranks as one of the most popular and in-demand programming languages, used by over 50% of professional developers worldwide. This widespread adoption makes SQL proficiency a valuable skill when evaluating candidates for technical positions.

This article provides hiring managers with a comprehensive set of SQL interview questions designed to assess candidates’ database knowledge and practical SQL skills, from basic queries to advanced database concepts.

What is SQL?

SQL (Structured Query Language) is a standardized programming language specifically designed for managing and manipulating relational databases. It allows developers to create, read, update, and delete data stored in database systems like MySQL, PostgreSQL, SQL Server, and Oracle.

SQL serves as the universal language for database operations, enabling developers to:

  • Query data from single or multiple tables
  • Insert, update, and delete records
  • Create and modify database structures
  • Manage user permissions and database security
  • Optimize database performance through indexing

Top 10 Must-Ask SQL Interview Questions

1. What is the difference between WHERE and HAVING clauses?

This question tests understanding of SQL query structure and filtering mechanisms.

Example Answer: “WHERE clause filters rows before grouping and cannot use aggregate functions. HAVING clause filters groups after GROUP BY and can use aggregate functions. Example: SELECT department, COUNT(*) FROM employees WHERE salary > 50000 GROUP BY department HAVING COUNT(*) > 5”

2. Explain the different types of SQL JOINs.

JOIN operations are fundamental to working with relational databases.

Example Answer:

  • “INNER JOIN: Returns only matching rows from both tables
  • LEFT JOIN: Returns all rows from left table, matching rows from right
  • RIGHT JOIN: Returns all rows from right table, matching rows from left
  • FULL OUTER JOIN: Returns all rows from both tables
  • CROSS JOIN: Returns Cartesian product of both tables”

3. What is the difference between DELETE, DROP, and TRUNCATE?

This tests understanding of data removal operations and their implications.

Example Answer: “DELETE removes specific rows and can use WHERE clause, logs each deletion, and can be rolled back. TRUNCATE removes all rows quickly, resets identity columns, but can’t use WHERE. DROP removes the entire table structure and data permanently.”

4. Explain database normalization and its forms.

Normalization is crucial for efficient database design.

Example Answer: “Normalization eliminates data redundancy and improves data integrity. 1NF: Atomic values, no repeating groups. 2NF: 1NF + no partial dependencies. 3NF: 2NF + no transitive dependencies. BCNF: 3NF + every determinant is a candidate key.”

5. What are indexes and how do they improve performance?

Understanding indexes is essential for database optimization.

Example Answer: “Indexes are data structures that improve query performance by creating shortcuts to table data. They speed up SELECT operations but slow down INSERT/UPDATE/DELETE. Types include clustered (physical data order), non-clustered (logical pointers), and composite (multiple columns).”

6. Explain ACID properties in database transactions.

ACID properties ensure database reliability and consistency.

Example Answer:

  • “Atomicity: Transaction is all-or-nothing
  • Consistency: Database remains valid after transaction
  • Isolation: Concurrent transactions don’t interfere
  • Durability: Committed changes persist permanently”

7. What is the difference between UNION and UNION ALL?

This tests knowledge of set operations in SQL.

Example Answer: “UNION combines results from multiple queries and removes duplicates, requiring extra processing. UNION ALL combines results without removing duplicates, making it faster. Both require same number of columns with compatible data types.”

8. How do you handle NULL values in SQL?

NULL handling is a common source of bugs and performance issues.

Example Answer: “Use IS NULL or IS NOT NULL for comparisons, never = or !=. Functions like ISNULL(), COALESCE(), or NULLIF() handle NULL values. NULLs in calculations return NULL, and they’re ignored in aggregate functions except COUNT(*).”

9. What are stored procedures and their advantages?

Stored procedures are important for complex database operations.

Example Answer: “Stored procedures are precompiled SQL code stored in the database. Advantages: better performance, code reusability, enhanced security, reduced network traffic, centralized business logic. Disadvantages: database-specific, harder debugging, version control challenges.”

10. Explain the concept of database locks and deadlocks.

Understanding locking mechanisms is crucial for concurrent database operations.

Example Answer: “Locks prevent data conflicts in concurrent transactions. Types: Shared (read), Exclusive (write), Intent locks. Deadlock occurs when two transactions wait for each other’s locks. Solutions: timeout, deadlock detection, proper transaction ordering, shorter transactions.”

Advanced SQL Concepts to Explore

For senior developer positions, consider these additional topics:

  • Window Functions: ROW_NUMBER(), RANK(), LAG(), LEAD()
  • Common Table Expressions (CTEs): Recursive and non-recursive
  • Query Optimization: Execution plans, query hints
  • Database Security: SQL injection prevention, user permissions
  • Data Warehousing: ETL processes, OLAP vs OLTP

Practical Assessment Tips

When interviewing SQL candidates:

  1. Provide real scenarios: Use actual business problems requiring SQL solutions
  2. Test optimization skills: Ask about query performance improvements
  3. Include data modeling: Assess ability to design efficient database schemas
  4. Check debugging abilities: Present problematic queries to troubleshoot
  5. Evaluate best practices: Discuss security, performance, and maintainability

Conclusion

SQL proficiency is essential for most development roles involving data persistence and retrieval. These interview questions help evaluate candidates’ understanding of both fundamental concepts and practical application of SQL in real-world scenarios.

The best SQL developers combine technical knowledge with problem-solving skills, understanding not just how to write queries but how to write efficient, maintainable, and secure database code that scales with business needs.

Consider using Interview Zen’s technical interview platform to create comprehensive SQL assessments and observe candidates’ problem-solving approaches in real-time during database-focused interviews.