TOP 20 SQL interview questions for 2025

 

TOP 20 SQL interview questions for 2025

Hey Candidates here are the Top 20 SQL, that will help you crack the SQL interview

1. What is SQL?

SQL (Structured Query Language) is a standard language for accessing and manipulating databases. It is used for querying, updating, and managing data stored in a relational database.

2. What are the differences between SQL and MySQL?

• SQL is a standard language for accessing and manipulating databases, while MySQL is a relational database management system (RDBMS) that uses SQL as its standard database language.
• SQL is a query language, and MySQL is software for managing databases.
• SQL commands are used in various database systems, whereas MySQL is a specific implementation of an RDBMS.

3. What are DDL and DML commands? Give examples.

• DDL (Data Definition Language) commands are used to define the database structure. Examples include CREATE, ALTER, and DROP.
• DML (Data Manipulation Language) commands are used for managing data within the database. Examples include SELECT, INSERT, UPDATE, and DELETE.

4. Write a query to retrieve all records from a table.

SELECT * FROM table_name;

5. How do you Join 2 tables

Tables can be joined using various types of joins, such as INNER JOIN, LEFT JOIN, RIGHT JOIN, 

and FULL JOIN. Here's an example using INNER JOIN:
SELECT *
FROM table1
INNER JOIN table2 ON table1.common_column = table2.common_column;

6. What is the difference between INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN?

• INNER JOIN returns records that have matching values in both tables.
• LEFT JOIN returns all records from the left table and matched records from the right table. If no match, NULL values are returned for columns of the right table.
• RIGHT JOIN returns all records from the right table and matched records from the left table. If no match, NULL values are returned for columns of the left table.
• FULL JOIN returns all records when there is a match in either left or right table. If there is no match, the result is NULL from the side where there is no match.

7. Explain the concept of normalization. 

Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. It involves dividing large tables into smaller tables and defining relationships between them. Common forms of normalization are 1NF, 2NF, 3NF, and BCNF.

8. What is indexing? How does it improve query performance? 

Indexing involves creating a data structure (index) that improves the speed of data retrieval operations on a database table. Indexes provide a quick lookup for matching rows in a table based on indexed columns, thus reducing the time required for data access.

9. Write a query to find the second highest salary from an Employee table.

SELECT MAX(salary) AS SecondHighestSalary
FROM Employee
WHERE salary < (SELECT MAX(salary) FROM Employee);

10. What is a primary key and a foreign key?
A primary key is a column (or a combination of columns) that uniquely identifies each row in a table.
A foreign key is a column (or a combination of columns) that establishes a relationship between two 

tables by referencing the primary key of another table.

11. Explain the difference between DELETE and TRUNCATE.

DELETE is a DML command used to delete specific rows from a table based on a condition.

 It can be rolled back.

TRUNCATE is a DDL command used to remove all rows from a table, resetting it to an empty state. 

It cannot be rolled back and is faster than DELETE.

12. What is a subquery? Provide an example.
A subquery is a query within another query. It is used to return data that will be used in the main query 

as a condition.

SELECT employee_name
FROM employees
WHERE department_id = (SELECT department_id FROM departments
WHERE department_name = 'Sales');

13. How do you handle transactions in SQL?
Transactions are handled using the following commands:
BEGIN TRANSACTION: Starts a transaction.
COMMIT: Saves all changes made during the transaction.
ROLLBACK: Reverts all changes made during the transaction.
14. What are stored procedures and functions?
A stored procedure is a set of SQL statements that can be executed as a single unit. It can perform 
actions such as data modification and return multiple values.
A function is a set of SQL statements that perform a specific task and return a single value. 
Functions are typically used for calculations and data manipulation.

15. Explain the concept of ACID properties in databases.
ACID stands for Atomicity, Consistency, Isolation, and Durability. These properties ensure reliable
 processing of database transactions:
Atomicity: Ensures that all operations within a transaction are completed; if not, the transaction
 is aborted.
Consistency: Ensures that the database remains in a consistent state before and after a transaction.
Isolation: Ensures that transactions are isolated from each other, preventing concurrent 
transactions from interfering with each other.
Durability: Ensures that once a transaction is committed, it remains permanent, even in the case 
of a system failure.
16. What is the difference between a VIEW and a TABLE?
A TABLE is a database object that stores data in rows and columns.
A VIEW is a virtual table based on the result set of a SQL query. It does not store data 
physically but provides a way to simplify complex queries and enhance security by restricting access 
to specific data.
17. How do you optimize SQL queries?
Use indexes to speed up data retrieval.
Avoid using SELECT * and only select required columns.
Use JOINs instead of subqueries when possible.
Write efficient WHERE clauses to filter data early.
Analyze query execution plans to identify performance bottlenecks.
18. What is the difference between DISTINCT and GROUP BY?
DISTINCT is used to remove duplicate rows from a result set.
GROUP BY is used to group rows that have the same values in specified columns and perform
 aggregate functions (e.g., COUNT, SUM) on them.

19. Explain the concept of database normalization.
Normalization is the process of organizing data in a database to reduce redundancy and improve 
data integrity. It involves dividing large tables into smaller tables and defining relationships between 
them. Common forms of normalization are 1NF, 2NF, 3NF, and BCNF.

20. What is the purpose of the ORDER BY clause?
The ORDER BY clause is used to sort the result set of a query by one or more columns. 
It can be used to sort data in ascending (ASC) or descending (DESC) order.

BEST LUCK !!!