SQL Interview Preparation: Essential Q&A for Beginners
Structured Query Language (SQL) is one of the most in-demand skills for freshers entering software development, database management, or data analysis roles. Almost every technical interview for backend or full-stack developer positions includes SQL questions. As a fresher, having clarity on SQL fundamentals and practicing real-world queries will help you stand out.
In this blog, we’ll cover essential SQL interview questions and answers for beginners to boost your confidence and prepare you for success.
1. What is SQL?
Answer: SQL (Structured Query Language) is the standard language used to interact with databases. It allows users to create, read, update, and delete data (CRUD operations). SQL also supports advanced tasks like creating tables, managing users, and defining relationships between data.
2. What are the different types of SQL statements?
Answer:
-
DDL (Data Definition Language):
CREATE,ALTER,DROP -
DML (Data Manipulation Language):
INSERT,UPDATE,DELETE -
DQL (Data Query Language):
SELECT -
DCL (Data Control Language):
GRANT,REVOKE -
TCL (Transaction Control Language):
COMMIT,ROLLBACK,SAVEPOINT
3. What is the difference between SQL and MySQL?
Answer:
-
SQL is a query language used to manage and manipulate data in databases.
-
MySQL is a database management system (DBMS) that uses SQL to store and manage data.
4. What is a primary key in SQL?
Answer: A primary key uniquely identifies each record in a table. It does not allow duplicate or NULL values. Example:
CREATE TABLE Students (
ID INT PRIMARY KEY,
Name VARCHAR(50)
);
5. What is a foreign key?
Answer: A foreign key links two tables by referencing the primary key of another table. It establishes a relationship between them.
6. What is the difference between WHERE and HAVING clause?
Answer:
-
WHERE filters rows before grouping.
-
HAVING filters groups after aggregation.
Example:
SELECT Course, COUNT(*)
FROM Students
GROUP BY Course
HAVING COUNT(*) > 5;
7. What is a JOIN in SQL?
Answer: JOINs combine rows from two or more tables based on related columns. Types of JOINs:
-
INNER JOIN
-
LEFT JOIN
-
RIGHT JOIN
-
FULL JOIN
Example:
SELECT Students.Name, Courses.CourseName
FROM Students
INNER JOIN Courses ON Students.CourseID = Courses.CourseID;
8. What is the difference between UNION and UNION ALL?
Answer:
-
UNION: Combines results and removes duplicates.
-
UNION ALL: Combines results including duplicates.
9. What are indexes in SQL?
Answer: An index improves the speed of data retrieval from a table. It works like an index in a book. Example:
CREATE INDEX idx_name ON Students(Name);
10. What is normalization?
Answer: Normalization is the process of organizing data to reduce redundancy and improve efficiency. Common normal forms are:
-
1NF (Eliminate repeating groups)
-
2NF (Eliminate partial dependencies)
-
3NF (Eliminate transitive dependencies)
11. What is denormalization?
Answer: Denormalization is the process of adding redundancy to improve performance by reducing joins.
12. What is a subquery in SQL?
Answer: A subquery is a query inside another query. Example:
SELECT Name
FROM Students
WHERE ID IN (SELECT StudentID FROM Results WHERE Marks > 80);
13. What is the difference between DELETE, TRUNCATE, and DROP?
Answer:
-
DELETE: Removes specific rows, can use WHERE.
-
TRUNCATE: Removes all rows, but keeps table structure.
-
DROP: Removes entire table including structure.
14. What is a view in SQL?
Answer: A view is a virtual table created from a query. Example:
CREATE VIEW StudentView AS
SELECT Name, Course FROM Students;
15. What is the difference between CHAR and VARCHAR?
Answer:
-
CHAR(n): Fixed length, pads with spaces if shorter.
-
VARCHAR(n): Variable length, saves space.
16. What are aggregate functions in SQL?
Answer: Functions that perform calculations on multiple rows:
-
COUNT() -
SUM() -
AVG() -
MIN() -
MAX()
17. What is a stored procedure?
Answer: A stored procedure is a set of SQL statements stored in the database. Example:
CREATE PROCEDURE GetStudents()
AS
SELECT * FROM Students;
18. What is the difference between clustered and non-clustered index?
Answer:
-
Clustered index: Sorts and stores rows physically.
-
Non-clustered index: Creates a separate index structure with pointers.
19. What is ACID property in SQL?
Answer:
-
Atomicity: All or nothing execution.
-
Consistency: Ensures data integrity.
-
Isolation: Transactions are independent.
-
Durability: Changes are permanent after commit.
20. What are transactions in SQL?
Answer: A transaction is a group of operations executed as a single unit. SQL uses BEGIN, COMMIT, and ROLLBACK for transaction control.
Conclusion
Preparing for an SQL interview Quesions and answers requires a balance of theory and hands-on practice. As a beginner, focus on writing queries, understanding normalization, and practicing joins. These 20 essential Q&A will help you confidently answer SQL interview questions and showcase your database skills.
With practice, you’ll be able to handle real-world data problems and impress your interviewer with clear, practical knowledge.
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