10 Most Common SQL Interview Questions and How to Answer Them​

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You want to stand out in sql interview sessions for high-paying companies. Focus on mastering essential sql interview questions and understanding advanced sql concepts. Real sql interview questions often test your grasp of database concepts, coding, optimization, and sql queries. The most frequently asked sql interview questions for 2025 include:

• ACID properties in database systems

• SQL commands and the role of primary keys

• Difference between inner join and outer join

• How to find duplicate rows in a database

• WHERE vs HAVING clauses in sql queries

• GROUP BY and ORDER BY usage

• SQL aggregate functions

• Constraints and indexes in database design

• Database normalization concepts

• How to get the second-highest salary using sql queries

Practice explaining your reasoning, use examples from sql coding interviews, and review performance and optimization questions. Mastering these queries and answers gives you an edge in sql courses and helps you optimize slow sql query performance.

1. Explain the ACID Properties in SQL Databases

Understanding the ACID properties is essential for anyone preparing for sql interview questions. Interviewers often ask you to explain these properties because they form the foundation of reliable sql database systems. ACID stands for Atomicity, Consistency, Isolation, and Durability. Each property ensures that your sql transactions maintain data integrity and reliability.

Here is a quick overview of the ACID properties and their practical implications:

You will see these properties in action every time you execute a sql transaction. For example, when you transfer money between two accounts, the database uses atomicity to make sure both the debit and credit happen together. Consistency checks that the total amount remains correct. Isolation prevents other sql queries from seeing the transaction until it finishes. Durability guarantees that the transaction stays saved, even if the system crashes right after you commit.

Sample Answer

When you answer this question in a sql interview, you should define each ACID property and explain its importance. For example, you can say:

“ACID properties in sql databases ensure reliable transaction processing. Atomicity means a transaction is all-or-nothing. Consistency keeps the database valid before and after transactions. Isolation prevents transactions from interfering with each other. Durability makes sure committed changes stay saved, even after a crash.”

Interview Tip

💡 In your sql interview, use real-world examples to show your understanding. For instance, describe how ACID properties protect data during online purchases or bank transfers. This approach demonstrates both your technical knowledge and your ability to apply concepts to practical scenarios.

2. SQL Commands and Primary Key

SQL commands form the backbone of every database operation. You use these commands to create, modify, and manage data in your database. In any sql interview, you will likely face questions about the main types of sql commands and the role of the primary key. Understanding these concepts helps you answer sql interview questions with confidence.

The main types of sql commands include:

• Data Definition Language (DDL): Commands like CREATE, ALTER, and DROP define and modify the structure of database objects.

• Data Manipulation Language (DML): Commands such as SELECT, INSERT, UPDATE, and DELETE manage data within tables.

• Data Control Language (DCL): Commands like GRANT and REVOKE control access to data in the database.

• Transaction Control Language (TCL): Commands such as COMMIT, ROLLBACK, and SAVEPOINT manage transactions in sql.

A primary key is a critical concept in sql. It uniquely identifies each record in a table. You must ensure that primary keys always have unique values and never contain NULLs. Each table can have only one primary key, but it can consist of one or more columns.

Here are some common ways to define a primary key in sql:

1. Create a primary key on a single column during table creation:

   1	CREATE TABLE Persons (ID int NOT NULL, PRIMARY KEY (ID));

2. Add a primary key to an existing table:

   1	ALTER TABLE Persons ADD PRIMARY KEY (ID);

3. Define a primary key on multiple columns:

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   CREATE TABLE Persons ( ID int NOT NULL, LastName varchar(255) NOT NULL, CONSTRAINT PK_Person PRIMARY KEY (ID, LastName) );

Sample Answer

When you answer sql interview questions about sql commands and primary keys, explain the main command types and describe how a primary key works. For example:

“SQL commands include DDL, DML, DCL, and TCL. The primary key uniquely identifies each row in a table. It must be unique and cannot be NULL. You can define a primary key on one or more columns, but each table can have only one primary key.”

Interview Tip

⚡ In your interview, avoid common mistakes such as allowing duplicate or NULL values in primary keys. Always follow best practices for naming and keep your key design simple. Show your understanding by explaining why data integrity depends on proper use of primary keys in every database.

3. Difference Between INNER JOIN and OUTER JOIN

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Understanding the difference between inner join and outer join is essential for any sql interview. Interviewers often ask you to explain how these joins work and when to use each type. You will see these joins in many sql interview questions because they help you combine data from multiple tables in a database.

Joins let you retrieve related data from two or more tables. You use joins to answer complex questions about your database. The most common types of joins include INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL OUTER JOIN. Each join type returns different results based on how the tables relate to each other.

Here is a table that summarizes the main types of joins in sql:

You use INNER JOIN when you want only the rows that exist in both tables. For example, if you have a customers table and an orders table, INNER JOIN gives you only the customers who have placed orders. LEFT JOIN returns all customers, even those who have not placed any orders. RIGHT JOIN does the opposite, showing all orders, even if there is no matching customer. FULL OUTER JOIN combines all rows from both tables, matching where possible and filling in NULLs where there is no match.

Sample Answer

You can answer this question in a sql interview by saying:

“The difference between inner join and outer join is that INNER JOIN returns only the rows with matching values in both tables. OUTER JOIN includes rows even if there is no match. LEFT JOIN returns all rows from the left table, RIGHT JOIN returns all rows from the right table, and FULL OUTER JOIN returns all rows from both tables, matching where possible.”

Interview Tip

📝 In your interview, use simple examples to show how each join works. Draw a quick diagram or write a short sql query. For example, use INNER JOIN to find students enrolled in classes, and LEFT JOIN to list all students, even those not enrolled. This approach shows you understand both the concept and its practical use in a database.

4. Find Duplicate Rows in SQL Interview

Duplicate data can cause major issues in any database. During a sql interview, you may face questions about how to find duplicate rows using sql. Interviewers want to see if you know how to keep data clean and accurate. You should know several effective ways to spot duplicates in a table.

Here are three common sql methods to find duplicate rows:

1. GROUP BY with HAVING
   You can group records by the columns you want to check for duplicates. Then, use the HAVING clause to filter groups with more than one row.

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   SELECT name, description, price, currency, COUNT(*) FROM products GROUP BY name, description, price, currency HAVING COUNT(*) > 1;

   This query helps you quickly see which values appear more than once in your database.

2. SELF JOIN
   You can join the table to itself to find rows with matching values in non-primary key columns.

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   SELECT DISTINCT p1.name, p1.description, p1.price, p1.currency FROM products p1 JOIN products p2 ON p1.name = p2.name AND p1.description = p2.description AND p1.price = p2.price AND p1.id <> p2.id;

   This method works well when you want to compare all columns except the unique identifier.

3. ROW_NUMBER Window Function
   You can assign a row number to each record within a group of duplicates. Then, select rows where the row number is greater than one.

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   WITH duplicated_products AS ( SELECT *, ROW_NUMBER() OVER(PARTITION BY name, description, price, currency ORDER BY id) AS row_num FROM products ) SELECT DISTINCT name, description, price, currency FROM duplicated_products WHERE row_num > 1;

   This approach gives you flexibility and works well in modern sql databases.

Sample Answer

You might answer sql interview questions like this:

“To find duplicate rows in a database, I use GROUP BY with HAVING to group and filter duplicates. I also use a self join to match rows with the same values except for the primary key. In modern sql, I use the ROW_NUMBER function to identify and filter duplicates. These methods help keep the database clean and reliable.”

Interview Tip

💡 In your interview, explain why removing duplicates matters for data quality. Show that you understand how each sql method works and when to use it. Practice writing these queries before your sql interview to build confidence.

5. WHERE vs HAVING in SQL Interview Questions

Understanding the difference between WHERE and HAVING is essential for any sql interview. Many sql interview questions test your ability to filter data correctly in a database. You need to know when to use each clause and how they affect your sql queries.

You use WHERE and HAVING to filter data, but they work at different stages of a sql query. Here is a quick breakdown:

• WHERE filters individual rows before any aggregation happens.

• HAVING filters groups after aggregation, especially when you use aggregate functions like COUNT, SUM, or AVG.

• WHERE cannot filter on grouped or aggregated data, while HAVING operates on the results of grouped data.

Consider this example. You want to find products in a database with more than one order:

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SELECT product_id, COUNT(*) AS order_count
FROM orders
WHERE status = 'completed'
GROUP BY product_id
HAVING COUNT(*) > 1;

In this query, WHERE filters orders with a status of ‘completed’ before grouping. HAVING then filters groups where the order count is greater than one. This approach ensures you only see products with multiple completed orders.

You often see group by and having used together in sql interview questions. Interviewers want to know if you understand how to filter data at both the row and group level. Mastering this concept helps you write efficient and accurate sql queries for any database.

Sample Answer

You can answer sql interview questions about WHERE vs HAVING like this:

“WHERE filters rows before any grouping or aggregation in sql. HAVING filters groups after aggregation, usually with aggregate functions. For example, I use WHERE to select orders from a specific customer, and HAVING to find customers with more than five orders in the database.”

Interview Tip

📝 In your interview, explain the order of operations in a sql query. Use simple examples to show how WHERE and HAVING work together. Practice writing queries that use both clauses, especially with group by and having. This skill shows you understand how to manage data at different stages in the database.

6. GROUP BY and ORDER BY in SQL

Understanding how to use GROUP BY and ORDER BY in sql helps you answer many sql interview questions with confidence. These clauses let you organize and present data from your database in meaningful ways. You often see them together in sql queries, especially when you need to analyze or report on large sets of data.

You use GROUP BY to combine rows that share the same values in specified columns. This clause works best when you want to apply aggregate functions like SUM(), AVG(), or COUNT() to grouped data. For example, you might want to see the total sales for each product in your database.

ORDER BY sorts the results of your sql query. You can arrange data in ascending (ASC) or descending (DESC) order based on one or more columns. This clause helps you present information clearly, such as showing the highest sales first or listing customers alphabetically.

Here are the key differences between GROUP BY and ORDER BY in sql:

• GROUP BY groups rows with the same values in selected columns.

• You use GROUP BY with aggregate functions to summarize data.

• ORDER BY sorts the result set by specified columns, either ascending or descending.

• You can use both clauses together to group data and then sort the grouped results.

Consider this example. You want to see the number of orders for each customer, sorted by the highest order count:

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SELECT customer_id, COUNT(*) AS order_count
FROM orders
GROUP BY customer_id
ORDER BY order_count DESC;

This query groups orders by customer_id, counts the number of orders for each customer, and sorts the results so the customer with the most orders appears first.

You often use GROUP BY and ORDER BY together in sql interview questions. Interviewers want to see if you understand how to organize and present data from a database. Practice writing queries that use both clauses to prepare for your next sql interview.

Sample Answer

You can answer sql interview questions about GROUP BY and ORDER BY like this:

“GROUP BY lets you group rows with the same values and use aggregate functions to summarize data. ORDER BY sorts the results by specified columns in ascending or descending order. You often use both together to group and then sort data in a sql database.”

Interview Tip

📝 In your interview, write clear sql queries that use GROUP BY and ORDER BY. Explain why you chose each clause and how they help you analyze data. Use examples from real database scenarios, such as sales reports or customer lists, to show your understanding.

7. SQL Aggregate Functions

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SQL aggregate functions help you analyze large sets of data in your database. You often see these functions in sql interview questions because they allow you to summarize and extract key insights from your tables. When you use aggregate functions, you can quickly answer business questions and make data-driven decisions.

Here are the most commonly used SQL aggregate functions and what they do:

• MIN() returns the smallest value in a column.

• MAX() returns the largest value in a column.

• COUNT() gives you the number of rows in a set.

• SUM() calculates the total sum of a numeric column.

• AVG() finds the average value of a numeric column.

You use these functions in many real-world scenarios. For example, you can analyze sales data by product category to see which items generate the most revenue. Teachers use aggregate functions to calculate average grades per subject and identify areas for improvement. Website administrators aggregate traffic data to find the most visited pages, which helps with resource planning. HR managers analyze salary distributions across departments to understand compensation structures.

A typical SQL query using aggregate functions looks like this:

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SELECT department, AVG(salary) AS average_salary
FROM employees
GROUP BY department;

This query helps you see the average salary for each department in your database. You can use similar queries to answer many business questions during a sql interview.

Sample Answer

You might answer sql interview questions about aggregate functions like this:

“SQL aggregate functions such as COUNT, SUM, AVG, MIN, and MAX help you summarize data in a database. For example, you can use COUNT to find the number of orders, SUM to calculate total sales, and AVG to see the average salary in each department. These functions are essential for data analysis and reporting.”

Interview Tip

💡 In your interview, explain how you use aggregate functions to solve real business problems. Give examples from sales, education, or HR. Practice writing queries that combine aggregate functions with GROUP BY to show your understanding of database analysis.

8. SQL Constraints and Indexes

SQL constraints and indexes play a vital role in maintaining data integrity and boosting performance in your database. In many sql interview questions, you will face topics about how these features work and why they matter. Understanding them helps you answer questions with confidence and shows your expertise in sql.

You use constraints to enforce rules on your database tables. These rules ensure that your data stays accurate and reliable. Indexes help you speed up data retrieval, making your queries run faster. However, you must use them wisely because too many indexes can slow down write operations.

Here is a table summarizing the most common types of sql constraints and indexes you might discuss in a sql interview:

You will often hear these common questions in a sql interview:

• What are some SQL constraints you know?

• What are SQL indexes and why are they important?

SQL constraints include NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, CHECK, and DEFAULT. You use these to keep your database consistent and prevent invalid data. SQL indexes are special structures that help your queries find data faster. You should add indexes to columns you search often, but avoid them on small tables or columns that change frequently.

Sample Answer

You can answer sql interview questions about constraints and indexes like this:

“SQL constraints are rules that keep your database accurate. Examples include PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, CHECK, and DEFAULT. SQL indexes help your queries run faster by allowing quick data lookup. You should use indexes on columns you search often, but avoid too many on tables that get updated a lot.”

Interview Tip

💡 In your interview, explain how constraints protect your data and how indexes improve performance. Give examples from real projects, such as using a UNIQUE constraint to prevent duplicate emails or adding an index to speed up customer searches. This shows you understand both theory and practice.

9. Database Normalization in SQL Interview

Database normalization is a key concept you will encounter in many sql interview questions. Interviewers want to see if you understand how to organize data in a sql database to reduce redundancy and improve data integrity. Normalization involves structuring your tables and columns so that each piece of information appears only once, making your database easier to maintain and update.

You achieve normalization by following a series of normal forms. Each normal form builds on the previous one, adding more rules to ensure your sql database stays efficient and consistent. Here are the main normal forms you should know for any sql interview:

1. First Normal Form (1NF): Make sure each table contains only atomic values, unique rows, and unique column names. The order of data does not matter.

2. Second Normal Form (2NF): Ensure the table meets 1NF and that every non-prime attribute depends on the entire primary key, removing partial dependencies.

3. Third Normal Form (3NF): Satisfy 2NF and remove transitive dependencies, so non-prime attributes do not depend on other non-prime attributes.

4. Boyce-Codd Normal Form (BCNF): Go beyond 3NF by making sure every non-trivial functional dependency has a superkey on the left side.

5. Fourth Normal Form (4NF): Achieve BCNF and eliminate multi-valued dependencies.

6. Fifth Normal Form (5NF): Reach 4NF and remove all join dependencies.

Normalization offers several benefits for your sql database. The table below highlights how it improves data integrity and reduces redundancy:

Sample Answer

You can answer sql interview questions about normalization like this:

“Database normalization is the process of organizing tables and columns in a sql database to reduce redundancy and improve data integrity. I follow normal forms such as 1NF, 2NF, and 3NF to ensure each table stores only relevant data. This makes updates easier and keeps the database consistent.”

Interview Tip

💡 In your interview, explain how normalization helps prevent duplicate data and keeps your sql database accurate. Use examples of splitting a large table into smaller ones to show your understanding. Practice describing each normal form and why it matters for real-world databases.

10. Get the Second-Highest Salary in SQL

Interviewers often ask you to get the second-highest salary from a table. This question tests your ability to write efficient SQL queries and understand ranking logic. You will see this topic in many top SQL interview lists. You should know several ways to solve this problem, as each method works best in different scenarios.

Here are five practical ways you can use to find the second-highest salary:

1. Subquery Method
   Use a subquery to exclude the highest salary and select the next maximum value.

   1	SELECT MAX(salary) FROM employee WHERE salary NOT IN (SELECT MAX(salary) FROM employee);

2. Self-Join Method
   Join the table with itself to compare salary values.

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   SELECT e1.salary FROM employee e1 JOIN employee e2 ON e1.salary < e2.salary GROUP BY e1.salary ORDER BY e1.salary DESC LIMIT 1;

3. DISTINCT and ORDER BY
   Select distinct salaries, order them, and pick the second row.

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   SELECT DISTINCT salary FROM employee ORDER BY salary DESC LIMIT 1,1;

4. NOT EXISTS Method
   Use a subquery to find the salary where no higher salary exists.

   1 2	SELECT salary FROM employee e1 WHERE NOT EXISTS (SELECT 1 FROM employee e2 WHERE e2.salary > e1.salary);

5. Window Function Method
   Use RANK() to assign ranks and select the salary with rank 2.

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   SELECT salary FROM ( SELECT salary, RANK() OVER (ORDER BY salary DESC) as rank FROM employee ) sub WHERE rank = 2;

Sample Answer

You can answer this interview question by saying:

“To get the second-highest salary in SQL, I use methods like subqueries, self-joins, DISTINCT with ORDER BY, NOT EXISTS, or window functions. Each approach works for different database systems and table structures. For example, I often use RANK() for large tables because it is efficient and easy to read.”

Interview Tip

💡 Practice writing each query before your interview. Explain why you choose a method based on table size and database type. If asked, show how to create an empty table with the same structure using CREATE TABLE new_table AS SELECT * FROM employee WHERE 1=0;. This shows you understand both query logic and table design.

Preparing for these 10 SQL interview questions gives you a strong foundation for any database role. Practice writing and explaining SQL queries using platforms like Interview Query, LeetCode, and HackerRank:

• Speak clearly about SQL concepts in your interview.

• Show your understanding of database design, query optimization, and practical application.

• Keep learning and stay updated with SQL trends to stand out in any SQL interview.

Candidates who communicate SQL concepts well and demonstrate data cleaning skills often succeed in interviews.

Recommended reading

• SQL Optimization Course
• SQL optimization knowledge
• Database industry knowledge

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