Top 15 SQL Interview Questions for Your Next Interview

SQL (Structured Query Language) is one of the most in-demand skills for freshers aiming to break into the IT and data fields. As a foundational technology for database management, SQL knowledge is essential for roles like data analysts, database administrators, and software developers. 

This article deepens into the most frequently asked SQL interview questions for freshers, helping you prepare effectively.

What is SQL?

SQL (Structured Query Language) is a standardized programming language used for managing and manipulating relational databases. Here’s a concise breakdown:

Core Purpose:

• Used to communicate with and control databases
• Handles tasks like storing, retrieving, updating, and deleting data
• Manages database structures (tables, views, procedures)

Key Functions:

1. Query data using SELECT statements
2. Update records with INSERT, UPDATE, DELETE
3. Create and modify database structures
4. Set user permissions on tables/views

SQL is essential for:

• Database Administrators
• Data Analysts
• Backend Developers
• Business Intelligence roles

Why Is SQL Important for Freshers?

Understanding why SQL is essential can boost your confidence during interviews. SQL is pivotal in managing and querying relational databases, enabling companies to handle massive amounts of data efficiently. Recruiters test SQL skills to evaluate a candidate’s ability to interact with and manage data, a critical skill in today’s data-driven world. 

Proficiency in SQL can set you apart from other freshers and open doors to lucrative career opportunities.

Basic SQL Interview Questions for Freshers

Interviewers often focus on fundamental SQL concepts for freshers to evaluate their foundational knowledge. These questions test your understanding of basic commands, keys, and database operations.

What is SQL?

SQL, or Structured Query Language, is used to interact with relational databases. It allows users to create, read, update, and delete database records.

What are the different types of SQL commands?

SQL commands are categorized into the following types:

• DDL (Data Definition Language): CREATE, ALTER, DROP
• DML (Data Manipulation Language): INSERT, UPDATE, DELETE
• DQL (Data Query Language): SELECT
• TCL (Transaction Control Language): COMMIT, ROLLBACK, SAVEPOINT
• DCL (Data Control Language): GRANT, REVOKE

What is a primary key?

A primary key is a column or set of columns uniquely identifying each table record. It does not allow NULL values.

Example:

CREATE TABLE Students (

   StudentID INT PRIMARY KEY,

   Name VARCHAR(50),

   Age INT

);

What is a foreign key?

A foreign key is a column in one table that refers to the primary key in another table, establishing a relationship between the two tables.

Example:

CREATE TABLE Orders (

   OrderID INT PRIMARY KEY,

   StudentID INT,

   FOREIGN KEY (StudentID) REFERENCES Students(StudentID)

);

What is a database?

A database is an organized data collection that can be accessed, managed, and updated using SQL.

Intermediate SQL Interview Questions for Freshers

Once you’ve mastered the basics, interviewers will test your ability to handle more complex queries and database scenarios. These questions require a deeper understanding of SQL operations and concepts.

What is the difference between DELETE and TRUNCATE?

• DELETE: Removes specific rows from a table using a condition. It can be rolled back.
• TRUNCATE: Removes all rows from a table and is faster but cannot be rolled back.

What are joins in SQL? Explain the types.

Joins combine rows from two or more tables based on a related column. Types include:

• INNER JOIN: Returns matching rows.
• LEFT JOIN: Returns all rows from the left table and matching rows from the right.
• RIGHT JOIN: Returns all rows from the right table and matching rows from the left.
• FULL JOIN: Returns rows when there is a match in either table.

Example:

SELECT Students.Name, Orders.OrderID

FROM Students

INNER JOIN Orders ON Students.StudentID = Orders.StudentID;

What is normalization? Explain its types.

Normalization is the process of organizing data to reduce redundancy and improve integrity. Types include:

• 1NF (First Normal Form): Eliminates duplicate columns.
• 2NF (Second Normal Form): Ensures all non-key attributes depend on the primary key.
• 3NF (Third Normal Form): Removes transitive dependencies.

What are indexes in SQL?

Indexes improve the speed of data retrieval. Types include:

• Clustered Index: Reorders the table’s physical data.
• Non-clustered Index: Maintains a separate structure for index storage.

What is a subquery?

A subquery is a query nested inside another query.

Example:

SELECT Name

FROM Students

WHERE StudentID IN (SELECT StudentID FROM Orders);

Advanced SQL Interview Questions for Freshers

Interviewers often include complex scenarios and advanced features for advanced roles or positions requiring more profound SQL knowledge. These questions test your ability to solve real-world database problems.

What are stored procedures?

Stored procedures are precompiled SQL statements that can be executed repeatedly to perform specific tasks.

Example:

CREATE PROCEDURE GetStudentOrders

AS

BEGIN

   SELECT * FROM Orders WHERE StudentID = 1;

END;

What is the difference between UNION and UNION ALL?

• UNION: Combines results from two queries and removes duplicates.
• UNION ALL: Combines results without removing duplicates.

What are the constraints in SQL?

Constraints enforce rules on data in a table. Examples:

• NOT NULL: Ensures a column cannot have a NULL value.
• UNIQUE: Ensures all values in a column are unique.
• CHECK: Ensures values meet a condition.

Explain the concept of transactions in SQL.

A transaction is a sequence of operations performed as a single logical unit of work. It follows the ACID properties:

• Atomicity: Ensures all tasks are completed.
• Consistency: Ensures the database remains consistent.
• Isolation: Transactions operate independently.
• Durability: Changes are permanent after a transaction is committed.

Example:

BEGIN TRANSACTION;

   INSERT INTO Orders (OrderID, StudentID) VALUES (1, 101);

   COMMIT;

What are triggers in SQL?

Triggers are SQL code automatically executed in response to specific events on a table, such as INSERT, UPDATE, or DELETE.

Example:

CREATE TRIGGER AfterInsert

ON Students

AFTER INSERT

AS

BEGIN

   PRINT ‘A new student record has been added’;

END;

Tips to Prepare for SQL Interviews

Preparing for an SQL interview requires both theoretical knowledge and practical application. Follow these tips to enhance your preparation and stand out during the interview process.

• Understand the basics thoroughly: Familiarize yourself with foundational SQL commands and concepts.
• Practice SQL queries: Use online platforms to practice.
• Work on projects: Create small database projects to gain practical experience.
• Learn about performance optimization: Understand indexing and query optimization techniques.
• Stay updated: SQL standards and best practices evolve; keep yourself informed.

Preparing for SQL interviews as a fresher can seem challenging, but mastering the basics, practicing queries, and understanding real-world applications will give you an edge. Use this guide as a reference to strengthen your SQL skills and land your dream job.

FAQs on SQL Interview Questions for Freshers

What are the main types of SQL commands?

SQL commands are grouped into DDL (Data Definition Language), DML (Data Manipulation Language), DCL (Data Control Language), and TCL (Transaction Control Language).

What’s the difference between DELETE and TRUNCATE?

DELETE removes specific rows and can be rolled back. TRUNCATE removes all rows, resets the identity counter, is faster, and can’t be rolled back.

Explain PRIMARY KEY and FOREIGN KEY constraints.

PRIMARY KEY uniquely identifies each record in a table. FOREIGN KEY establishes relationships between tables by referencing a PRIMARY KEY in another table.

What is the difference between INNER and LEFT JOIN?

INNER JOIN returns matching records from both tables. LEFT JOIN returns all records from the left table and matching records from the right table.

What are SQL aggregate functions?

Typical aggregate functions include COUNT, SUM, AVG, MAX, and MIN. They perform calculations on multiple rows and return a single value.

Explain the WHERE vs HAVING clause.

WHERE filters individual rows before grouping. HAVING filters groups after GROUP BY clause. HAVING works with aggregate functions, WHERE doesn’t.

What are SQL indexes and their importance?

Indexes improve query performance by providing faster data retrieval. They create data structure that speeds up searching and sorting operations.

How to remove duplicates in SQL?

Use DISTINCT keyword in SELECT statement or GROUP BY clause. For removing duplicate rows, use the DELETE with ROW_NUMBER() function.

What is SQL normalization?

Normalization organizes database tables to minimize redundancy. Standard forms are 1NF, 2NF, and 3NF, each addressing specific data integrity issues.

Explain SQL subqueries.

Subqueries (nested queries) are queries within another query. They can be used in SELECT, FROM, and WHERE clauses to perform complex operations.

What’s the difference between UNION and UNION ALL?

UNION combines results and removes duplicates. UNION ALL combines results, including duplicates. UNION ALL performs better as it skips duplicate removal.

How to handle NULL values in SQL?

Use IS NULL or IS NOT NULL in WHERE clause. COALESCE or ISNULL functions replace NULL with specified values.

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