SQL developers are in high demand in today’s job market. But even with so many opportunities available, confidently answering SQL interview questions can be one of the most challenging obstacles for many freshers like you.
So, to help you land the perfect job for you, we’ll guide you through some of the commonly asked basic SQL interview questions and answers for freshers as well as technical ones. You can use these SQL interview questions for freshers with answers as reference to practice and gain confidence before you next interview.
So, let’s begin.
Top 40 SQL Interview Questions Freshers in 2024
Here are 40 of the most frequently asked SQL interview questions for freshers and how to answer them:
Q1. What is SQL?
- Briefly explain that SQL stands for Structured Query Language (a programming language used to interact with relational databases)
- Highlight its functionalities like data retrieval, manipulation, and management.
Q2. What are the different types of SQL statements?
- Mention the main categories:
- DDL (Data Definition Language) for creating and altering database structures (CREATE, ALTER, DROP)
- DML (Data Manipulation Language) for working with data (SELECT, INSERT, UPDATE, DELETE)
- DCL (Data Control Language) for managing user access (GRANT, REVOKE).
Q3. Explain the concept of a relational database.
- Define a relational database as a collection of interconnected tables with rows and columns.
- Emphasise the importance of relationships established through foreign keys, ensuring data consistency.
Q4. Differentiate between SELECT * and SELECT column_name1, column_name2…
- Explain that SELECT * retrieves all columns from a table, while specifying column names allows for selective retrieval, improving performance and clarity.
Q5. What are the different types of joins in SQL?
- Briefly explain the main join types:
- INNER JOIN retrieves matching records from two tables
- LEFT JOIN includes all records from the left table and matching records from the right
- RIGHT JOIN vice versa
- FULL JOIN includes all records from both tables.
Q6. How can you filter data in SQL?
- Mention the WHERE clause as the primary way to filter data based on specific conditions.
- Briefly explain comparison operators like =, <>, >, <, LIKE, etc.
Q7. What do you understand by ORDER BY clause?
- Explain that the ORDER BY clause sorts the retrieved data in ascending or descending order based on a specified column.
Q8. How can you group data in SQL?
- Introduce the GROUP BY clause, which groups data based on a column and allows applying aggregate functions like SUM, COUNT, AVG, etc.
Q9. Explain the difference between WHERE and HAVING clauses.
- Explain that the WHERE clause filters data before grouping, while HAVING filters groups after applying aggregate functions.
Q10. What are subqueries in SQL?
- Define subqueries as nested queries that return a result set used within another SQL statement (SELECT, INSERT, UPDATE, DELETE).
Q11. What is a view in SQL?
- Explain that a view is a virtual table based on a predefined SQL query. It simplifies complex queries and enhances data security by controlling access to underlying tables.
Q12. How can you handle duplicate data in SQL?
- Mention the DISTINCT keyword to eliminate duplicates in the result set.
- You can also discuss using the UNIQUE constraint on a column or combination of columns to prevent duplicates during data insertion.
Q13. What are the different data types in SQL?
- Briefly explain common data types like CHAR, VARCHAR, INT, FLOAT, DATE, etc., highlighting their usage for storing different kinds of data.
Q14. Explain the concept of ACID properties in a database.
- Briefly define ACID properties:
- Atomicity (transactions are indivisible)
- Consistency (data adheres to all constraints)
- Isolation (transactions are isolated from each other)
- Durability (committed transactions persist).
Q15. How can you write a query to find the top 5 customers by total order amount?
- This is a practical question so be prepared to write a SQL query using functions like SUM and ORDER BY with a LIMIT clause to retrieve the desired results.
Q16. Explain the difference between a primary key and a foreign key.
- Define a primary key as a unique identifier for a table row, enforcing data integrity.
- Explain that a foreign key references a primary key in another table, establishing relationships and preventing orphaned data.
Q17. How can you combine results from multiple tables?
- Reiterate the different join types (INNER, LEFT, RIGHT, FULL) and explain how they’re used to combine data based on matching columns between tables.
Q18. What are aggregate functions in SQL? Provide some examples.
- Define aggregate functions that operate on entire groups of data. Give examples like:
- COUNT (number of rows)
- SUM (total of a column)
- AVG (average)
- MIN (smallest value)
- MAX (largest value).
Q19. Explain the concept of aliases in SQL.
- Describe aliases as temporary names assigned to tables or columns in a query, improving readability and avoiding lengthy names.
Q20. How can you update existing data in a table?
- Explain the UPDATE statement, specifying the table, the changes to be made (SET clause), and the condition for selecting the rows to update (WHERE clause).
Q21. What are NULL values in SQL, and how can you handle them?
- Define NULL values as representing the absence of data.
- Mention functions like ISNULL or COALESCE to check for NULL values and provide alternative values.
Q22. How can you write a query to find the difference between the highest and lowest salary in an employee table?
- Explain using aggregate functions like MAX and MIN, along with arithmetic operations, to calculate the difference.
Q23. Explain self-joins in SQL.
- Define self-joins as joining a table to itself based on a common column.
- Discuss their usage for hierarchical data or finding relationships within the same table.
Q24. What do you understand by indexes in a database?
- Explain indexes as data structures that improve query performance by speeding up data retrieval.
- Briefly mention different types of indexes.
Q25. How can you import data from a CSV file into a SQL table?
- The specific method might depend on the database system. Generally, explain that most database systems offer tools or commands to import data from external files.
Q26. What are stored procedures in SQL?
- Explain stored procedures as predefined SQL code blocks that can be reused with different parameters, promoting code modularity and maintainability.
Q27. Explain the concept of transactions in SQL.
- Define a transaction as a logical unit of work involving multiple database operations.
- Briefly mention ACID properties that ensure data consistency within transactions.
Q28. How can you write a query to find the average salary per department in an employee table?
- This is another practical question – explain using GROUP BY on the department column and the AVG function on the salary column to calculate the average per department.
Q29. What are some best practices for writing efficient SQL queries?
- Discuss practices like using proper indexing, avoiding unnecessary SELECT *, filtering data efficiently with WHERE clause, and optimising joins.
Q30. How can you secure your SQL queries against SQL injection attacks?
- Briefly explain SQL injection vulnerabilities and emphasise using parameterised queries where user input is separated from the main SQL statement, preventing malicious code execution.
Q31. What is a database?
- Define a database as a structured collection of data organised into tables, allowing for efficient storage, retrieval, and manipulation.
Q32. What are the different types of SQL commands? Briefly explain their purpose.
- Mention the main categories:
- DDL (Data Definition Language): CREATE, ALTER, DROP – for defining and modifying the structure of databases (tables, views, etc.).
- DML (Data Manipulation Language): SELECT, INSERT, UPDATE, DELETE – for working with data within tables (retrieving, adding, modifying, and removing).
- DCL (Data Control Language): GRANT, REVOKE – for managing user access and permissions to the database.
Q33. Explain the concept of a primary key.
- Define a primary key as a unique identifier for each row in a table. It enforces data integrity by ensuring no duplicate values exist for that column (or set of columns).
Q34. What are the benefits of using JOINs in SQL?
- Explain how JOINs allow combining data from multiple tables based on a shared column. This enables retrieving related information across tables, providing a more comprehensive view of the data.
Q35. How can you filter data in SQL? Provide an example.
- Introduce the WHERE clause as the primary way to filter data based on specific conditions.
- Give an example: SELECT * FROM customers WHERE city = ‘New York’; (This retrieves all rows from the “customers” table where the “city” column is equal to “New York”).
Q36. What do aggregate functions do in SQL? List some common examples.
- Define aggregate functions as those that operate on entire groups of data, returning a single summarised value.
- Mention common examples like COUNT (number of rows), SUM (total of a column), AVG (average), MIN (smallest value), and MAX (largest value).
Q37. Explain the difference between UNION and UNION ALL.
- Clarify that both UNION and UNION ALL combine the results of two or more SELECT statements. However, UNION removes duplicate rows, while UNION ALL includes all rows, even duplicates.
Q38. How can you write a query to find the total number of employees in each department?
- Explain using GROUP BY on the department column and the COUNT function to calculate the total number of employees per department.
Q39. What are some best practices for writing clear and readable SQL code?
- Discuss practices like using meaningful table and column names, adding comments to explain complex logic, and proper indentation for better code structure.
Q40. Explain how you would approach writing a query to find customers who haven’t placed any orders in the last 6 months.
- This is a scenario-based question. Break it down:
- Identify relevant tables (customers and orders).
- Use a JOIN to connect them (likely a LEFT JOIN to include all customers).
- Filter the results using a WHERE clause to identify customers with no orders in a specific date range (consider date manipulation functions).
Go through these SQL interview questions and answers for freshers thoroughly and practice multiple times. And during your interview, be confident, explain your thought process, and show your ability to apply your SQL knowledge to solve problems.
All the best!
FAQs on SQL Interview Questions for Freshers
Q1. What are some common SQL interview questions for beginners?
Ans: Here are some common SQL interview questions for beginners:
- Basic database & SQL concepts (tables, queries, data types)
- Filtering data (WHERE clause)
- Joining tables (INNER JOIN, LEFT JOIN)
- Aggregate functions (COUNT, SUM, AVG)
- Writing simple queries (retrieving data, filtering)
Q2. How to prepare for SQL interview for freshers?
Ans: Here’s how to prep for a SQL interview as a fresher:
- Understand the fundamentals like databases, SQL basics (DML, DDL), data types, and simple queries.
- Use tutorials, practice problems, and online platforms to solidify your understanding.
- Focus on core concepts like joins (INNER, LEFT), filtering (WHERE), and functions (COUNT, SUM, AVG).
- Practice writing queries for specific tasks like finding customer trends or filtering data.
- Learn about writing clear, readable, and efficient SQL code.
- Familiarise yourself with frequently asked interview questions for beginners.
Q3. How to explain SQL project in interview?
Ans: Here’s how to explain an SQL project in an interview:
- Explain the project’s goal and the problem it addressed.
- Mention the specific tasks you performed (data analysis, query writing, etc.).
- Show the SQL concepts you used (joins, functions, etc.).
- Mention any improvements or outcomes achieved through your project.
Q4. How to explain SQL skills in an interview?
Ans: To explain your SQL skills in an interview, follow these steps:
- Briefly mention your experience with SQL (courses, projects, personal use).
- Show your understanding of core concepts like DML (Data Manipulation Language) for data retrieval and modification (SELECT, INSERT, UPDATE, DELETE).
- Mention your knowledge of tables, columns, data types, and basic database design principles.
- Highlight your ability to write queries using WHERE clauses for filtering, JOINs for combining data, and basic aggregate functions (COUNT, SUM, AVG).
