SQL Query Interview Questions and Answers (2025)

SQL has been around for decades, but it’s still one of the most in-demand skills for coding interview prep today. Almost every company, no matter the industry, depends on data. And SQL is the language used to store, query, and manage that data. That’s why you’ll find SQL at the center of so many interview questions.

If you’re preparing for a SQL interview, the right preparation will help you stand out. Interviewers aren’t just looking for someone who can type out a query. They want to know if you can think through problems, design efficient solutions, and optimize performance. That’s exactly what SQL query interview questions are built to test.

By the end of this guide, you’ll be prepared to handle both simple and advanced coding questions with confidence. This guide offers in-depth answers, practical examples, and strategies to prepare for coding interviews that will help you master the most common SQL query interview questions.

Why SQL Is a Popular Choice in Coding Interviews

SQL is one of the most widely used technologies in the world, and that’s exactly why it shows up in interviews so often. Whether you’re applying for a role as a software engineer, data analyst, or data scientist, SQL is almost always part of the evaluation.

Every industry, from finance to e-commerce to healthcare, relies on relational databases. If you know SQL, you can work with data anywhere.

While programming languages change, relational databases have stayed consistent. Interviewers know that if you’re comfortable with SQL, you’ll be able to extract insights from data and support applications that depend on it.

SQL is important across roles. Developers need it to integrate with databases, analysts use it to find insights, and data scientists rely on it to prepare and filter datasets. That’s why it’s tested across so many positions.

You’ll face many SQL query interview questions because they reveal how well you can interact with data, regardless of your role. It’s one of the most reliable ways for interviewers to gauge whether you can think in terms of data, efficiency, and scalability.

Categories of SQL Query Interview Questions

The best way to prepare for SQL interviews is to understand the categories of questions you’ll encounter. Each category tests a different aspect of your knowledge, from basic queries to advanced optimization.

Here are the most common categories of SQL query interview questions:

Basic SQL syntax (SELECT, WHERE, ORDER BY)
Tests your ability to retrieve and filter data.
Joins and set operations
Includes INNER, LEFT, RIGHT, FULL JOIN, as well as UNION, INTERSECT, and EXCEPT.
Aggregate functions (COUNT, SUM, AVG, GROUP BY, HAVING)
Covers summarization and grouping of datasets.
Subqueries and nested queries
Used to solve problems where one query depends on the result of another.
Window functions (RANK, ROW_NUMBER, PARTITION BY)
Common in analytics-focused interviews for ranking, running totals, and moving averages.
Indexes and performance optimization
Tests whether you can write queries that scale.
Database schema design and normalization
Evaluates how well you can structure tables for efficiency and clarity.
Stored procedures and transactions
Important for backend and enterprise-level roles.
Advanced problems (recursive queries, CTEs, pivoting)
High-level challenges that push you to think beyond basics.
System-level SQL design questions
Asked in senior interviews to test architecture-level thinking.

By preparing across these categories, you’ll build a strong foundation. In the next section, we’ll start with the basics.

Basic SQL Query Interview Questions

Before diving into joins, subqueries, or window functions, interviewers want to make sure you’ve mastered the fundamentals. Many SQL query interview questions start with these basics before moving to complex problems.

1. What are the different types of SQL commands?

Explanation:

SQL commands are grouped into categories:

DDL (Data Definition Language): CREATE, ALTER, DROP.
DML (Data Manipulation Language): SELECT, INSERT, UPDATE, DELETE.
DCL (Data Control Language): GRANT, REVOKE.
TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT.

Sample Answer:

“SQL commands fall into categories like DDL for defining structure, DML for managing data, DCL for permissions, and TCL for transactions.”

2. How do you use SELECT with WHERE and ORDER BY?

Explanation:

SELECT retrieves data, WHERE filters it, and ORDER BY sorts it.

SELECT name, salary  

FROM Employees  

WHERE department = ‘HR’  

ORDER BY salary DESC;

Sample Answer:

“This query finds employees in HR and sorts them by salary in descending order.”

3. Difference between DISTINCT and GROUP BY

Explanation:

DISTINCT removes duplicates from the result set.
GROUP BY groups rows and is usually used with aggregates.

SELECT DISTINCT department FROM Employees;  

SELECT department, COUNT(*) FROM Employees GROUP BY department;

Sample Answer:

“DISTINCT eliminates duplicate rows, while GROUP BY groups rows to allow aggregate calculations.”

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

Explanation:

DELETE: Removes rows but keeps the table.
TRUNCATE: Removes all rows, resets storage, but keeps the table structure.
DROP: Deletes the table entirely.

DELETE FROM Employees WHERE id=5;  

TRUNCATE TABLE Employees;  

DROP TABLE Employees;

Sample Answer:

“DELETE removes specific rows, TRUNCATE clears the table, and DROP deletes the table itself.”

5. Explain NULL handling in SQL

Explanation:

NULL represents missing or unknown data.
Comparisons with NULL return unknown.
Use IS NULL or IS NOT NULL to check values.
Functions like COALESCE() or IFNULL() replace NULL with defaults.

SELECT name, COALESCE(salary, 0) as salary FROM Employees;

Sample Answer:

“In SQL, NULL means missing data. You can’t use = to compare with NULL—you need IS NULL. To handle NULLs in calculations, functions like COALESCE help.”

Takeaway: These fundamentals prove whether you can write basic queries correctly. Many SQL query interview questions begin here, then build toward joins, subqueries, and optimization.

Joins and Set Operations

One of the most important skills in SQL is combining data from multiple tables. Many SQL query interview questions focus on joins and set operations, because they test how well you can work with relational data.

1. Difference between INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN

INNER JOIN: Returns only the rows with matching values in both tables.
LEFT JOIN: Returns all rows from the left table and matching rows from the right. Non-matches are filled with NULLs.
RIGHT JOIN: Opposite of LEFT JOIN—keeps all rows from the right table.
FULL JOIN: Returns all rows from both tables, with NULLs where no match exists.

Example: Suppose you have two tables:

Employees (id, name, dept_id)  

Departments (id, dept_name)

— INNER JOIN

SELECT e.name, d.dept_name

FROM Employees e

INNER JOIN Departments d ON e.dept_id = d.id;

This query shows employees with valid department matches.

2. What is a SELF JOIN?

A SELF JOIN joins a table with itself. It’s useful for hierarchical data, like employees and managers.

SELECT e.name AS Employee, m.name AS Manager

FROM Employees e

LEFT JOIN Employees m ON e.manager_id = m.id;

This query lists employees and their managers.

3. UNION vs UNION ALL

UNION: Combines results of two queries and removes duplicates.
UNION ALL: Combines results and keeps duplicates.

SELECT name FROM Employees

UNION

SELECT dept_name FROM Departments;

4. INTERSECT and EXCEPT usage

INTERSECT: Returns rows that appear in both queries.
EXCEPT (or MINUS in some databases): Returns rows from the first query that are not in the second.

— Employees who are also department names

SELECT name FROM Employees

INTERSECT

SELECT dept_name FROM Departments;

Takeaway: Joins and set operations are the foundation of relational databases. Many SQL query interview questions in this category test whether you can combine, filter, and structure data effectively.

Aggregate Functions and GROUP BY

Aggregations let you summarize data, which is critical in real-world analytics. Interviewers often test your ability to group and filter results.

1. How do you use COUNT, SUM, AVG, MIN, MAX?

SELECT COUNT(*) AS total_employees,

       AVG(salary) AS avg_salary,

       SUM(salary) AS total_salary,

       MIN(salary) AS min_salary,

       MAX(salary) AS max_salary

FROM Employees;

Answer: Aggregates summarize columns. COUNT counts rows, SUM adds them, AVG calculates averages, and MIN/MAX find extremes.

2. Difference between WHERE and HAVING

WHERE filters rows before aggregation.
HAVING filters groups after aggregation.

SELECT dept_id, AVG(salary) as avg_salary

FROM Employees

GROUP BY dept_id

HAVING AVG(salary) > 60000;

3. Find the department with the highest average salary

SELECT dept_id, AVG(salary) as avg_salary

FROM Employees

GROUP BY dept_id

ORDER BY avg_salary DESC

LIMIT 1;

4. Find duplicate rows in a table

SELECT name, COUNT(*)

FROM Employees

GROUP BY name

HAVING COUNT(*) > 1;

Takeaway: Aggregates appear in almost every SQL interview. These SQL query interview questions check whether you know when to use WHERE vs HAVING and how to summarize data correctly.

Subqueries and Nested Queries

Subqueries let you solve complex problems by embedding one query inside another. They’re a favorite in interviews because they test logical thinking.

1. What is a correlated subquery?

A correlated subquery references a column from the outer query.

SELECT name, salary

FROM Employees e

WHERE salary > (

  SELECT AVG(salary)

  FROM Employees

  WHERE dept_id = e.dept_id

);

This finds employees earning above their department’s average.

2. EXISTS vs IN

IN checks if a value is in a list of values returned by a subquery.
EXISTS checks if a subquery returns any rows.

SELECT name

FROM Employees e

WHERE EXISTS (

  SELECT 1

  FROM Departments d

  WHERE d.id = e.dept_id

);

3. Scalar subqueries in SELECT clauses

A scalar subquery returns a single value.

SELECT name,

       (SELECT dept_name FROM Departments d WHERE d.id = e.dept_id) AS department

FROM Employees e;

Takeaway: Subqueries are versatile. Many SQL query interview questions focus on whether you know when to use correlated subqueries, EXISTS, and scalar queries.

Window Functions in SQL Query Interview Questions

Window functions are one of the most advanced and powerful features in SQL. They’re especially common in data analyst and data scientist interviews.

1. Difference between RANK, DENSE_RANK, and ROW_NUMBER

SELECT name, salary,

       RANK() OVER (ORDER BY salary DESC) as rank,

       DENSE_RANK() OVER (ORDER BY salary DESC) as dense_rank,

       ROW_NUMBER() OVER (ORDER BY salary DESC) as row_num

FROM Employees;

RANK: Leaves gaps after ties.
DENSE_RANK: No gaps after ties.
ROW_NUMBER: Assigns unique numbers, ignoring ties.

2. How to calculate running totals with window functions

SELECT name, salary,

       SUM(salary) OVER (ORDER BY id) as running_total

FROM Employees;

3. How to find the nth highest salary

SELECT DISTINCT salary

FROM Employees e1

WHERE 3 = (

  SELECT COUNT(DISTINCT salary)

  FROM Employees e2

  WHERE e2.salary >= e1.salary

);

Or with ROW_NUMBER:

SELECT salary

FROM (

  SELECT salary, ROW_NUMBER() OVER (ORDER BY salary DESC) as rn

  FROM Employees

) t

WHERE rn = 3;

4. Partitioning vs ordering in window functions

PARTITION BY divides results into groups.
ORDER BY defines the order of rows inside each partition.

SELECT dept_id, name, salary,

       RANK() OVER (PARTITION BY dept_id ORDER BY salary DESC) as dept_rank

FROM Employees;

Takeaway: These SQL query interview questions test if you can use advanced analytics features in SQL. Window functions are crucial for modern data roles.

Indexes and Performance Optimization

Performance is just as important as correctness. Interviewers often check if you can write queries that scale.

1. What are indexes and how do they work?

An index is like a lookup table for faster searches. Instead of scanning the whole table, the database jumps directly to rows using the index.

CREATE INDEX idx_emp_salary ON Employees(salary);

2. Clustered vs non-clustered indexes

Clustered index: Determines the physical order of rows in the table. One per table.
Non-clustered index: Separate structure pointing to rows. You can have many.

3. When can indexes slow down performance?

On small tables, indexes add overhead.
Too many indexes slow down INSERT, UPDATE, and DELETE operations because each index must be updated.

4. How do you analyze query execution plans?

Use the database’s EXPLAIN or EXPLAIN ANALYZE command. It shows whether your query uses indexes or performs full table scans.

EXPLAIN SELECT * FROM Employees WHERE salary > 60000;

Takeaway: These optimization-related SQL query interview questions show whether you think about efficiency, not just correctness. Strong candidates know how to balance indexes and query design.

Database Schema Design and Normalization

Schema design is one of the most important parts of SQL interviews. It shows whether you can structure databases that scale while avoiding redundancy.

1. What are the different normal forms (1NF, 2NF, 3NF, BCNF)?

Normalization organizes data to reduce redundancy.

1NF (First Normal Form): No repeating groups or arrays. Each column holds atomic values.
2NF (Second Normal Form): Must be in 1NF and all non-key attributes depend on the entire primary key (no partial dependency).
3NF (Third Normal Form): Must be in 2NF and no transitive dependencies (non-key attributes should not depend on other non-key attributes).
BCNF (Boyce-Codd Normal Form): Stricter than 3NF; every determinant must be a candidate key.

Sample Answer:

“In interviews, I’d explain that normalization improves consistency and prevents anomalies in insert, update, and delete operations.”

2. Why is denormalization sometimes used?

While normalization reduces redundancy, it can make queries slower due to multiple joins. Denormalization adds redundancy back in for performance reasons.

Example: Storing a customer’s current order count in their profile table instead of recalculating with every query.

3. Design a schema for an e-commerce site

Tables you might include:

Users (user_id, name, email)
Products (product_id, name, price, stock_qty)
Orders (order_id, user_id, order_date, status)
Order_Items (order_item_id, order_id, product_id, quantity, price)
Payments (payment_id, order_id, method, status)

Reasoning:

Orders and products have a many-to-many relationship, handled by Order_Items.
Normalization keeps data consistent. For example, product prices are stored in Products but copied into Order_Items to preserve historical accuracy.

Takeaway: These schema-related SQL query interview questions check if you can balance normalization with performance in real-world designs.

Stored Procedures, Transactions, and ACID

These concepts show whether you understand database reliability and maintainability, which is essential for backend roles.

1. What is a stored procedure and when should you use one?

A stored procedure is a precompiled SQL block saved in the database.

CREATE PROCEDURE GetEmployeeCount()

BEGIN

  SELECT COUNT(*) FROM Employees;

END;

Use cases:

Reusable business logic.
Improved performance since queries are precompiled.
Security by controlling access.

2. What are transactions?

A transaction is a group of SQL statements executed as a single unit. Either all succeed, or none do.

BEGIN;

UPDATE Accounts SET balance = balance – 100 WHERE id=1;

UPDATE Accounts SET balance = balance + 100 WHERE id=2;

COMMIT;

3. Explain ACID properties

Atomicity: All or nothing.
Consistency: Database moves from one valid state to another.
Isolation: Transactions don’t interfere with each other.
Durability: Once committed, changes are permanent.

4. Difference between COMMIT and ROLLBACK

COMMIT: Saves all changes made in the transaction.
ROLLBACK: Undoes all changes.

Takeaway:

Questions on stored procedures, transactions, and ACID are common because they test if you can build reliable, consistent systems.

Advanced SQL Query Interview Questions

These advanced SQL query interview questions are often asked for senior roles to test deep query knowledge.

1. What are Common Table Expressions (CTEs)?

A CTE is a temporary named result set used in complex queries.

WITH DeptSalary AS (

  SELECT dept_id, AVG(salary) as avg_salary

  FROM Employees

  GROUP BY dept_id

)

SELECT * FROM DeptSalary WHERE avg_salary > 60000;

2. How do recursive queries work?

Recursive CTEs allow you to query hierarchical data.

WITH RECURSIVE EmployeeHierarchy AS (

  SELECT id, name, manager_id

  FROM Employees

  WHERE manager_id IS NULL

  UNION ALL

  SELECT e.id, e.name, e.manager_id

  FROM Employees e

  JOIN EmployeeHierarchy h ON e.manager_id = h.id

)

SELECT * FROM EmployeeHierarchy;

3. How to pivot and unpivot tables?

PIVOT: Converts rows to columns.
UNPIVOT: Converts columns to rows.

— Example: Sales pivoted by month

SELECT product_id,

       SUM(CASE WHEN month=’Jan’ THEN sales ELSE 0 END) as JanSales,

       SUM(CASE WHEN month=’Feb’ THEN sales ELSE 0 END) as FebSales

FROM Sales

GROUP BY product_id;

4. What are windowed aggregates?

Windowed aggregates apply functions across partitions.

SELECT dept_id, name, salary,

       AVG(salary) OVER (PARTITION BY dept_id) as dept_avg

FROM Employees;

Takeaway: These advanced features test if you can solve real-world, enterprise-level problems with SQL.

Practice Section: Mock SQL Query Interview Questions

Here are some full-length practice questions with solutions:

1. Find the second highest salary in a company

SELECT MAX(salary)

FROM Employees

WHERE salary < (SELECT MAX(salary) FROM Employees);

2. Identify customers who purchased in consecutive months

SELECT DISTINCT c.customer_id

FROM Orders c1

JOIN Orders c2

  ON c1.customer_id = c2.customer_id

 AND DATE_TRUNC(‘month’, c1.order_date) = DATE_TRUNC(‘month’, c2.order_date – interval ‘1 month’);

3. Write a query to detect duplicate emails

SELECT email, COUNT(*)

FROM Users

GROUP BY email

HAVING COUNT(*) > 1;

4. Calculate a rolling 7-day average of sales

SELECT order_date,

       AVG(amount) OVER (ORDER BY order_date ROWS BETWEEN 6 PRECEDING AND CURRENT ROW) as rolling_avg

FROM Sales;

5. Retrieve employees with the highest salary per department

SELECT e.*

FROM Employees e

WHERE salary = (

  SELECT MAX(salary)

  FROM Employees

  WHERE dept_id = e.dept_id

);

Takeaway: Practicing mock problems helps you simulate interview scenarios and build confidence with SQL query interview questions.

Tips for Solving SQL Query Interview Questions

Success in SQL interviews isn’t just about writing correct queries. It’s also about clarity, structure, and efficiency.

Write clear, readable queries: Use proper indentation. Interviewers should be able to follow your logic easily.
Explain your thought process: Don’t just write code—walk through how you’re approaching the problem.
Use aliases: Short table aliases like e for Employees make queries easier to read.
Think about performance: Consider whether indexes exist and whether your query will scan entire tables unnecessarily.
Handle NULL values carefully: Remember that comparisons with NULL require IS NULL. Functions like COALESCE() can help.
Practice without autocomplete: Many interviews are done on shared docs or whiteboards. Get used to writing queries from memory.

By applying these strategies, you’ll approach SQL query interview questions with confidence and structure.

Wrapping Up

SQL is one of the few skills that cuts across almost every technical role. Whether you’re working as an engineer, analyst, or data scientist, SQL is a must-have.

The key takeaway is simple: mastering SQL query interview questions will give you the confidence to tackle both technical and analytical interviews. Don’t just memorize queries, but practice them, explain them out loud, and build real-world projects where you use SQL daily.

Stay consistent. The more you practice, the more second-nature SQL will feel. Explore our other guides, courses, and resources to continue building your skills.

SQL Query Interview Questions​ and Answers (2025)

Why SQL Is a Popular Choice in Coding Interviews

Categories of SQL Query Interview Questions

Basic SQL Query Interview Questions

1. What are the different types of SQL commands?

2. How do you use SELECT with WHERE and ORDER BY?

3. Difference between DISTINCT and GROUP BY

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

5. Explain NULL handling in SQL

Joins and Set Operations

1. Difference between INNER JOIN, LEFT JOIN, RIGHT JOIN, FULL JOIN

2. What is a SELF JOIN?

3. UNION vs UNION ALL

4. INTERSECT and EXCEPT usage

Aggregate Functions and GROUP BY

1. How do you use COUNT, SUM, AVG, MIN, MAX?

2. Difference between WHERE and HAVING

3. Find the department with the highest average salary

4. Find duplicate rows in a table

Subqueries and Nested Queries

1. What is a correlated subquery?

2. EXISTS vs IN

3. Scalar subqueries in SELECT clauses

Window Functions in SQL Query Interview Questions

1. Difference between RANK, DENSE_RANK, and ROW_NUMBER

2. How to calculate running totals with window functions

3. How to find the nth highest salary

4. Partitioning vs ordering in window functions

Indexes and Performance Optimization

1. What are indexes and how do they work?

2. Clustered vs non-clustered indexes

3. When can indexes slow down performance?

4. How do you analyze query execution plans?

1. What are the different normal forms (1NF, 2NF, 3NF, BCNF)?

2. Why is denormalization sometimes used?

3. Design a schema for an e-commerce site

1. What is a stored procedure and when should you use one?

2. What are transactions?

3. Explain ACID properties

4. Difference between COMMIT and ROLLBACK

Advanced SQL Query Interview Questions

1. What are Common Table Expressions (CTEs)?

2. How do recursive queries work?

3. How to pivot and unpivot tables?

4. What are windowed aggregates?

Practice Section: Mock SQL Query Interview Questions

1. Find the second highest salary in a company

2. Identify customers who purchased in consecutive months

3. Write a query to detect duplicate emails

4. Calculate a rolling 7-day average of sales

5. Retrieve employees with the highest salary per department

Tips for Solving SQL Query Interview Questions

Wrapping Up

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