🎓 Whether you're preparing for a job interview or just getting started with databases, mastering SQL is a must. This blog post covers the Top 40 SQL questions you’re likely to face, categorized by topic, with simple explanations and practice tips.

We’ve also included helpful visuals (like stickman-style diagrams) to make concepts easier to remember! Let’s dive in. 👇

---

🔹 BASICS OF SQL

1. What is SQL?

SQL (Structured Query Language) is a language used to communicate with and manage databases. It lets you insert, query, update, and delete data.

🧠 Think of it like talking to your database in a language it understands.

---

2. What is a database?

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

💡 Imagine a super-organized digital filing cabinet.

---

3. Types of SQL Commands:

• DDL (Data Definition Language) – CREATE, DROP, ALTER

• DML (Data Manipulation Language) – INSERT, UPDATE, DELETE

• DQL (Data Query Language) – SELECT

• DCL (Data Control Language) – GRANT, REVOKE

• TCL (Transaction Control Language) – COMMIT, ROLLBACK

🗂 Think of each as different tools in a toolbox.

---

4. CHAR vs. VARCHAR2:

• CHAR: Fixed-length (e.g., CHAR(10) uses all 10 characters).

• VARCHAR2: Variable-length (e.g., VARCHAR2(10) uses only what’s needed).

📏 Use VARCHAR2 for flexibility!

---

5. What is a Primary Key?

A primary key uniquely identifies each row in a table. It must be unique and not null.

---

6. What is a Foreign Key?

A foreign key links one table to another, enforcing referential integrity.

---

7. DEFAULT Constraint:

Automatically inserts a default value when none is provided.

🧃 Example: quantity INT DEFAULT 1

---

🔹 DATABASE DESIGN & NORMALIZATION

8. What is Normalization?

Process of organizing data to reduce redundancy.

🪜 Levels: 1NF → 2NF → 3NF

---

9. Denormalization:

Combining tables to improve read performance, even if it creates some redundancy.

🚀 Used in data-heavy or read-optimized systems.

---

🔹 SQL QUERIES & FUNCTIONS

10. What is a Query?

A SQL statement to fetch or manipulate data. Example:

sql: SELECT * FROM employees;

---

11. SQL Operators:

=, !=, >, <, LIKE, BETWEEN, IN, IS NULL

---

12. What is a View?

A virtual table created from a query.

sql: 
CREATE VIEW high_salary AS
SELECT name FROM employees WHERE salary > 50000;

---

🔹 CONSTRAINTS & JOINS

13. UNIQUE Constraint:

Ensures all values in a column are different.

---

14. Types of Joins:

• INNER JOIN: Matching rows in both tables.

• LEFT JOIN: All rows from left, and matched from right.

• RIGHT JOIN: All rows from right, and matched from left.

• FULL OUTER JOIN: All rows from both tables.

🧩 Joins connect tables like puzzle pieces!

---

15. INNER JOIN vs. OUTER JOIN:

• INNER: Only matched rows.

• OUTER: Includes unmatched rows (with NULLs).

---

🔹 AGGREGATIONS & SUBQUERIES

16. GROUP BY Clause:

Used to group rows by a column for aggregation.

sql: 

SELECT department, COUNT(*) FROM employees GROUP BY department;

---

17. Aggregate Functions:

• SUM(), AVG(), COUNT(), MAX(), MIN()

---

18. What is a Subquery?

A query inside another query.

SELECT name FROM employees WHERE salary > (SELECT AVG(salary) FROM employees);

---

🔹 FILTERING & INDEXING

19. WHERE vs. HAVING:

• WHERE: Filters rows before grouping.

• HAVING: Filters after GROUP BY.

---

20. Indexes:

Used to speed up data retrieval. Similar to an index in a book 📖

---

🔹 DATA MANIPULATION & OPTIMIZATION

21. DELETE vs. TRUNCATE:

• DELETE: Removes specific rows, can use WHERE.

• TRUNCATE: Removes all rows, faster, cannot rollback.

---

22. ORDER BY Clause:

Sorts results by one or more columns.

sql: 
SELECT name FROM employees ORDER BY salary DESC;

---

🔹 SQL vs NoSQL

23. Key Differences:

SQL	NoSQL
Relational DBs	Non-relational DBs
Structured schemas	Flexible schemas
ACID compliant	BASE (eventual consistency)

---

🔹 DATABASE OBJECTS & CONSTRAINTS

24. What is a Table?

A table is a collection of rows and columns that store data.

---

25. Types of Constraints:

• NOT NULL

• UNIQUE

• PRIMARY KEY

• FOREIGN KEY

• CHECK

• DEFAULT

---

26. Cursor:

A pointer to rows returned by a query. Used in procedural SQL for row-by-row operations.

---

27. Trigger:

A procedure that runs automatically when certain events happen in a table (e.g., INSERT).

---

🔹 SQL STATEMENTS

28. SELECT Statement:

Used to fetch data from a table.

---

29. NULL Values:

NULL represents missing or unknown data.

---

30. Stored Procedure:

Reusable SQL code block stored in the database.

---

🔹 DDL & DML

31. DDL vs DML:

DDL	DML
CREATE, DROP	INSERT, UPDATE

---

32. ALTER Command:

Modifies an existing table structure (add/remove column).

---

🔹 ADVANCED SQL CONCEPTS

33. Composite Primary Key:

Primary key made up of two or more columns.

---

34. Data Integrity:

Maintained using constraints, triggers, and proper schema design.

---

35. Stored Procedure Advantages:

• Faster performance

• Reusability

• Security

---

🔹 SET OPERATIONS & CASE HANDLING

36. UNION:

Combines rows from two queries (removes duplicates).

---

37. UNION vs UNION ALL:

• UNION: No duplicates

• UNION ALL: Includes duplicates

---

38. CASE Statement:

Used for conditional logic.

sql:
SELECT name,
  CASE
    WHEN salary > 50000 THEN 'High'
    ELSE 'Low'
  END AS SalaryLevel
FROM employees;

---

🔹 FUNCTIONS & SPECIAL OPERATIONS

39. Scalar Functions:

Return a single value. Examples: UPPER(), NOW(), LENGTH()

---

40. COALESCE():

Returns the first non-null value.

sql:
SELECT COALESCE(middle_name, 'N/A') FROM users;

---

📘 Bonus: SQL Practice Tips

🔹 Use a free SQL playground like SQLFiddle or DB-Fiddle
🔹 Create your own employee or student database and try out all the queries
🔹 Use Excel to simulate database tables
🔹 Practice explaining each query in your own words
🔹 Focus on writing clean, readable SQL

---

📌 Conclusion

Whether you’re a beginner or preparing for a data role, these top 40 SQL interview questions cover the essentials in a way that’s easy to understand and apply. Save this post, practice regularly, and you’ll master SQL in no time!

💬 Have any questions or want more example queries? Drop a comment below or reach out!

---

📌 Follow me on Youtube