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November 26, 2024

Must-Know questions on SQL for interview for Freshers 2025

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Questions on SQL for interview Must-Know for Freshers 2025

SQL (Structured Query Language) is a critical skill for developers, analysts, and data professionals, especially when it comes to interviews for entry-level jobs. Read more about Questions on SQL for interview Must-Know for Freshers 2025!

Questions on SQL for interview Must-Know for Freshers 2025

So as per this Questions on SQL for interview Must-Know for Freshers 2025 article, Whether you are a fresher preparing for an SQL interview or someone aiming to switch fields, having a solid understanding of key SQL concepts and queries is essential.

In this Questions on SQL for interview Must-Know for Freshers 2025 article, we’ll walk you through the must-know SQL interview questions for freshers in 2025, along with their answers and tips to help you ace your interview.

1. What is SQL and why is it important?

Answer:
So based on this Questions on SQL for interview Must-Know for Freshers 2025 article, SQL (Structured Query Language) is a standard programming language designed for managing and manipulating relational databases. It allows you to query databases, update records, insert new data, delete data, and perform administrative functions like creating and altering database structures.

So as guided in this Questions on SQL for interview Must-Know for Freshers 2025 article, SQL is important because it provides a means for businesses and organizations to interact with their data, making it an indispensable skill for any tech or data-driven role. Whether you are managing customer data, inventory, or performing analysis, SQL is at the heart of database management.

2. Explain the different types of SQL statements.

Answer:
So considering this Questions on SQL for interview Must-Know for Freshers 2025 article, SQL statements can be categorized into several types:

  • Data Query Language (DQL): Used to query the database and retrieve data.
    • Example: SELECT
  • Data Definition Language (DDL): Deals with the structure (schema) of the database.
    • Examples: CREATE, ALTER, DROP
  • Data Manipulation Language (DML): Used to manipulate the data in the database.
    • Examples: INSERT, UPDATE, DELETE
  • Data Control Language (DCL): Controls access to data in the database.
    • Examples: GRANT, REVOKE
  • Transaction Control Language (TCL): Manages the transactions in a database.
    • Examples: COMMIT, ROLLBACK, SAVEPOINT

3. What is a primary key? Can you give an example?

Answer:
So according to this Questions on SQL for interview Must-Know for Freshers 2025 article, A primary key is a unique identifier for a record in a database table. It ensures that each record can be uniquely identified and avoids duplicate entries. A primary key must contain unique values and cannot contain NULLs.

Example: If you have a table employees, the employee_id can be the primary key because each employee has a unique ID.

sql
CREATE TABLE employees (
employee_id INT PRIMARY KEY,
name VARCHAR(50),
position VARCHAR(50)
);

4. What are joins in SQL? Explain different types of joins.

Answer:
A join is a method of combining rows from two or more tables based on a related column. The most common types of joins are:

  • INNER JOIN: Returns rows that have matching values in both tables.
    • Example:
      sql
      SELECT * FROM employees
      INNER JOIN departments
      ON employees.department_id = departments.department_id;
  • LEFT JOIN (OUTER JOIN): Returns all rows from the left table, along with matching rows from the right table. If there is no match, NULL values will be returned for columns of the right table.
  • RIGHT JOIN (OUTER JOIN): Similar to LEFT JOIN but returns all rows from the right table.
  • FULL OUTER JOIN: Returns all rows when there is a match in either left or right table.

5. What is normalization? Explain its types.

Answer:
Normalization is the process of organizing data in a database to reduce redundancy and improve data integrity. The goal is to break down complex tables into smaller, simpler tables to ensure minimal duplication of data and make the database more efficient.

There are several normal forms:

  • 1st Normal Form (1NF): Ensures that all columns contain atomic values (no multiple values in a single column).
  • 2nd Normal Form (2NF): Achieved by eliminating partial dependency; all non-key attributes must depend on the entire primary key.
  • 3rd Normal Form (3NF): Ensures that all attributes are functionally dependent only on the primary key.

6. What is an index? Why is it used?

Answer:
An index is a database object that improves the speed of data retrieval operations on a table at the cost of additional space and slower write operations (insert, update, delete). An index is created on one or more columns of a table to allow for faster searching and sorting.

For example, creating an index on the employee_name column allows faster searching for employees by their name.

sql
CREATE INDEX idx_employee_name
ON employees (employee_name);

7. What are subqueries in SQL?

Answer:
A subquery is a query nested inside another query. It can be used in various SQL clauses like WHERE, FROM, or SELECT. Subqueries can return a single value, a list of values, or even a table.

Example:

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

8. Explain the difference between WHERE and HAVING clauses.

Answer:

  • WHERE: Filters rows before any grouping takes place (used for individual records).
  • HAVING: Filters rows after grouping (used for aggregated records like SUM, AVG, etc.).

Example:

sql
SELECT department_id, COUNT(*) AS employee_count
FROM employees
GROUP BY department_id
HAVING COUNT(*) > 5;

9. What is a foreign key?

Answer:
A foreign key is a column or a set of columns in a table that is used to establish a link between the data in two tables. It is a reference to the primary key in another table, ensuring referential integrity between the two tables.

Example:

sql
CREATE TABLE employees (
employee_id INT PRIMARY KEY,
department_id INT,
FOREIGN KEY (department_id) REFERENCES departments(department_id)
);

10. What is the difference between DELETE and TRUNCATE commands?

Answer:

  • DELETE: Removes rows from a table based on a condition and can be rolled back if the transaction is not committed.
  • TRUNCATE: Removes all rows from a table and cannot be rolled back. It is faster than DELETE because it does not log individual row deletions.

11. What are aggregate functions in SQL?

Answer:
Aggregate functions perform a calculation on a set of values and return a single result. Common aggregate functions include:

  • COUNT(): Counts the number of rows.
  • SUM(): Adds up values in a column.
  • AVG(): Calculates the average of values in a column.
  • MAX(): Returns the maximum value.
  • MIN(): Returns the minimum value.

Example:

sql
SELECT AVG(salary) FROM employees;

12. What is a view in SQL?

Answer:
A view is a virtual table that provides a way to encapsulate complex queries. It does not store data physically but provides a simplified view of the data stored in one or more tables.

Example:

sql
CREATE VIEW employee_salary_view AS
SELECT employee_name, salary
FROM employees
WHERE department_id = 1;

Conclusion About Questions on SQL for interview Must-Know for Freshers 2025

SQL is a vast field, and while these questions form the core of any SQL interview for freshers, it is essential to keep practicing and refining your skills. As technology evolves, new techniques and database management systems emerge, so staying updated is key.

By mastering these SQL concepts and being ready to answer real-world scenarios, you’ll be well-prepared to crack any SQL interview. Be sure to check out other learning resources and participate in SQL coding challenges to improve your proficiency.

If you are looking for more insights on SQL interview questions or other tech-related topics, stay tuned to our blog for the latest updates and tutorials. So this concludes the article about Questions on SQL for interview Must-Know for Freshers 2025!

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