SQL Interview Questions Every Developer Should Know

Structured Query Language (SQL) is a must-know for any software developer, data analyst, or database administrator. Whether you’re preparing for an entry-level role or a senior database engineer position, SQL interview questions often test not just theoretical knowledge but also practical problem-solving skills. In this article, we’ll cover the most commonly asked SQL interview questions, breaking down each topic with simple explanations, relatable analogies, and real-world examples.

1. What is SQL, and Why is it Important?

SQL (Structured Query Language) is used to communicate with relational databases. It allows users to create, manipulate, and retrieve data efficiently. Almost every modern application—whether a banking system, e-commerce site, or social media platform—relies on SQL to manage and process data.

Think of SQL as a language similar to English that lets you “talk” to databases. If a database is like a vast library, SQL queries are like asking a librarian for a specific book using clear instructions.

2. What are the Different Types of SQL Commands?

SQL commands are categorized into five main types:

• DDL (Data Definition Language) – Defines the structure of the database (e.g., CREATE, ALTER, DROP)
• DML (Data Manipulation Language) – Modifies data within tables (e.g., INSERT, UPDATE, DELETE)
• DQL (Data Query Language) – Retrieves data (e.g., SELECT)
• DCL (Data Control Language) – Controls access permissions (e.g., GRANT, REVOKE)
• TCL (Transaction Control Language) – Manages transactions (e.g., COMMIT, ROLLBACK)

Real-life analogy: If a database is a spreadsheet, DDL is like defining the columns, DML is like editing rows, and DQL is like searching for data.

3. What is the Difference Between SQL and NoSQL?

Feature	SQL (Relational) Databases	NoSQL (Non-relational) Databases
Structure	Tables with rows & columns	Flexible schema (documents, key-value, graph, etc.)
Scalability	Vertical scaling (adding more power to a single server)	Horizontal scaling (adding more servers)
Use Case	Structured data, complex queries	Unstructured or semi-structured data

Example: If SQL is like an Excel sheet with structured rows and columns, NoSQL is like a flexible JSON document where each entry can have different fields.

4. What is a Primary Key and Foreign Key?

Primary Key: Uniquely identifies a record in a table. Example:
CREATE TABLE Employees (
EmployeeID INT PRIMARY KEY,
Name VARCHAR(50),
Age INT

• );

Foreign Key: A field that links two tables together. Example:
CREATE TABLE Orders (
OrderID INT PRIMARY KEY,
EmployeeID INT,
FOREIGN KEY (EmployeeID) REFERENCES Employees(EmployeeID)

• );

Analogy: If a Primary Key is a passport number (unique to one person), a Foreign Key is like a visa stamp linking a passport to a country’s immigration database.

5. What is Normalization? Explain its Different Forms.

Normalization is the process of organizing data to eliminate redundancy and improve efficiency. The normal forms are:

• 1NF (First Normal Form): Ensures each column contains atomic (indivisible) values.
• 2NF (Second Normal Form): Removes partial dependencies (no non-key column should depend on part of a composite key).
• 3NF (Third Normal Form): Removes transitive dependencies (a non-key column should not depend on another non-key column).

Example: If a student database has a column storing multiple phone numbers as "12345, 67890", breaking them into separate rows achieves 1NF.

6. What is an Index in SQL?

An index is used to speed up data retrieval. Think of it as a book’s index, allowing quick lookup of topics without flipping through every page.

CREATE INDEX idx_employee_name ON Employees (Name);

Types of indexes:

• Clustered Index: Sorts data physically in the table (only one per table).
• Non-Clustered Index: Stores pointers to data (multiple allowed per table).

7. What are Joins in SQL? Explain Different Types with Examples.

Joins are used to retrieve data from multiple tables based on relationships.

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

Example:

SELECT Employees.Name, Orders.OrderID
FROM Employees
INNER JOIN Orders ON Employees.EmployeeID = Orders.EmployeeID;

8. What is the Difference Between DELETE, TRUNCATE, and DROP?

• DELETE: Removes specific rows but retains the table structure.
• TRUNCATE: Removes all rows but retains the table structure.
• DROP: Deletes the entire table from the database.

Example:

DELETE FROM Employees WHERE EmployeeID = 101; – Deletes one row
TRUNCATE TABLE Employees; – Deletes all rows
DROP TABLE Employees; – Deletes the entire table

9. What is a Stored Procedure?

A stored procedure is a set of SQL statements saved for reuse.

CREATE PROCEDURE GetEmployeeDetails @EmpID INT
AS
BEGIN
SELECT * FROM Employees WHERE EmployeeID = @EmpID;
END;

To execute:

EXEC GetEmployeeDetails 101;

Benefits: Improves performance, ensures reusability, and enhances security.

10. Common SQL Interview Mistakes to Avoid

• Forgetting to use WHERE in DELETE statements, leading to full table deletion.
• Using SELECT * instead of specifying required columns.
• Ignoring indexing, which can slow down queries.
• Not handling NULL values properly, leading to unexpected results.
• Misunderstanding GROUP BY, causing incorrect aggregations.

Conclusion

Mastering SQL isn’t just about memorizing queries—it’s about understanding how data works in real-world applications. By practicing these interview questions and concepts, you’ll not only ace your SQL interviews but also become a more efficient developer. Keep practicing SQL queries, understand how databases function, and apply best practices to write optimized, error-free queries.

Got an upcoming SQL interview? Start solving real-world problems on platforms like LeetCode and HackerRank to sharpen your skills!