SQL is one of the most important skills for Data Analyst interviews.

However, interviewers rarely test only syntax. They want to know whether you can retrieve the right data, identify patterns and solve realistic business problems.

This guide covers the major SQL topics you should prepare before a Data Analyst interview.

SQL Fundamentals

• What is SQL?
• What is a primary key?
• What is a foreign key?
• What is NULL?
• What is the difference between DELETE, DROP and TRUNCATE?
• What is DISTINCT?
• What is an alias?
• What is a CASE statement?
• What is a subquery?
• What is a CTE?

JOIN Questions

• What is an INNER JOIN?
• What is a LEFT JOIN?
• What is a RIGHT JOIN?
• What is a FULL OUTER JOIN?
• What is a SELF JOIN?
• What happens when a JOIN creates duplicate rows?
• How do you identify unmatched records?
• How would you join three tables?
• What is the difference between JOIN and UNION?
• How do you prevent accidental row multiplication?

Aggregation Questions

• What does GROUP BY do?
• Difference between WHERE and HAVING?
• How do COUNT and COUNT(*) differ?
• How do you calculate average revenue per customer?
• How would you calculate monthly sales?
• How would you identify the highest-selling product?
• How would you calculate percentage contribution?
• How would you calculate a rolling average?

Window Functions

• What is a window function?
• Difference between RANK and DENSE_RANK?
• Difference between ROW_NUMBER and RANK?
• How do you find the second-highest salary?
• How do you find the top three products per category?
• How do you calculate a running total?
• How do you calculate month-over-month growth?
• How do you identify the first transaction for every customer?

Real-World SQL Questions

• Find customers who haven’t purchased in 90 days.
• Find duplicate customer records.
• Find the top 5 products by revenue.
• Calculate customer retention.
• Calculate average order value.
• Find customers with more than three purchases.
• Calculate daily active users.
• Find the highest revenue month.
• Calculate conversion rate.
• Compare current-month revenue with previous-month revenue.
• Identify customers whose spending increased.
• Find employees earning more than their department average.
• Calculate cumulative revenue.
• Identify the first and latest transaction for each customer.

How to Approach SQL Interview Problems

Don’t immediately start writing SQL.

Understand → Define → Identify tables → Identify joins → Filter → Aggregate → Validate

For example, if asked to calculate conversion rate, first clarify what counts as a visitor, what counts as a conversion, the time period, the population and whether duplicate events are possible.

The SQL comes after the metric definition.

SQL Interview Tip

After writing your query, mentally validate: Are NULLs handled? Can JOINs duplicate rows? Is the aggregation at the correct level? Are date boundaries correct? Does the result make business sense?

This is what turns SQL knowledge into analytical thinking.

Final Preparation

SQL interviews can range from simple filtering questions to complex business problems involving multiple tables and window functions.

Practice both syntax and business-oriented SQL problems.

For a structured collection of SQL and other Data Analyst interview questions, see the Data Analyst Interview Mastery Playbook, which covers 500+ questions across the complete interview process.