Crack your SQL interview with these Top 20 SQL Interview Questions and Answers. Includes detailed explanations, examples, and key differences for freshers and experienced candidates.

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🔹 1. What is SQL?

Answer:
SQL (Structured Query Language) is a standard language used to manage and manipulate databases.

Example:

SELECT * FROM employees;

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🔹 2. What is the difference between DELETE, TRUNCATE, and DROP?

Command	Description	Rollback Possible	Affects Structure
DELETE	Deletes selected rows	Yes	No
TRUNCATE	Deletes all rows quickly	No	No
DROP	Deletes the entire table	No	Yes

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🔹 3. What are 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)

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Top 20 SQL Interview Questions and Answers

🔹 4. What is a primary key?

Answer:
A primary key is a column (or set of columns) that uniquely identifies each row in a table.

Example:

CREATE TABLE students (
    student_id INT PRIMARY KEY,
    name VARCHAR(50)
);

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🔹 5. What is a foreign key?

Answer:
A foreign key in one table points to a primary key in another table to establish a relationship.

Example:

FOREIGN KEY (student_id) REFERENCES students(student_id)

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🔹 6. What is the difference between WHERE and HAVING?

Clause	Use Case	Works On
WHERE	Filters rows before grouping	Rows
HAVING	Filters after GROUP BY	Aggregated data

Example:

SELECT dept, COUNT(*) FROM employees
GROUP BY dept
HAVING COUNT(*) > 5;

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Top 20 SQL Interview Questions and Answers

🔹 7. What are JOINS in SQL?

Answer:
JOINS combine rows from two or more tables.

Join Type	Description
INNER JOIN	Matches rows in both tables
LEFT JOIN	All rows from left + matched from right
RIGHT JOIN	All rows from right + matched from left
FULL JOIN	All matched + unmatched rows from both

Example:

SELECT e.name, d.name
FROM employees e
INNER JOIN departments d ON e.dept_id = d.id;

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🔹 8. What is the difference between UNION and UNION ALL?

Keyword	Duplicates	Faster
UNION	Removes	No
UNION ALL	Keeps	Yes

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🔹 9. What is normalization?

Answer:
Normalization is a process to reduce data redundancy and improve data integrity.

Normal Forms:

• 1NF: Atomic columns
• 2NF: No partial dependency
• 3NF: No transitive dependency

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Top 20 SQL Interview Questions and Answers

🔹 10. What is a subquery?

Answer:
A subquery is a query nested inside another query.

Example:

SELECT name FROM students
WHERE id IN (SELECT student_id FROM enrollments);

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🔹 11. What is the difference between IN and EXISTS?

• IN: Checks values in a list or subquery.
• EXISTS: Checks if subquery returns at least one row.

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🔹 12. What is an index?

Answer:
An index improves query performance by allowing faster search and retrieval.

Example:

CREATE INDEX idx_name ON employees(name);

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🔹 13. What is a view?

Answer:
A view is a virtual table based on a SQL query.

Example:

CREATE VIEW active_employees AS
SELECT * FROM employees WHERE status = 'active';

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Top 20 SQL Interview Questions and Answers

🔹 14. What is a stored procedure?

Answer:
A stored procedure is a precompiled group of SQL statements.

Example:

CREATE PROCEDURE GetEmployees()
AS
BEGIN
    SELECT * FROM employees;
END;

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🔹 15. What is the difference between CHAR and VARCHAR?

Type	Fixed/Variable	Storage
CHAR(n)	Fixed length	Always n bytes
VARCHAR(n)	Variable length	Up to n bytes

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🔹 16. What are aggregate functions in SQL?

Answer:
Aggregate functions perform calculations on multiple rows.

Function	Purpose
SUM()	Adds values
AVG()	Average
COUNT()	Count rows
MAX()	Maximum value
MIN()	Minimum value

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🔹 17. What is a constraint?

Answer:
Constraints enforce rules on data columns.

• NOT NULL
• UNIQUE
• PRIMARY KEY
• FOREIGN KEY
• CHECK
• DEFAULT

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🔹 18. What is the difference between BETWEEN and IN?

• BETWEEN checks for a range.
• IN checks for specific values.

Example:

-- BETWEEN
SELECT * FROM students WHERE age BETWEEN 18 AND 25;

-- IN
SELECT * FROM students WHERE city IN ('Delhi', 'Mumbai');

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Top 20 SQL Interview Questions and Answers

🔹 19. What is a transaction in SQL?

Answer:
A transaction is a unit of work that is either fully completed or fully failed.

Commands:

• BEGIN TRANSACTION
• COMMIT
• ROLLBACK

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🔹 20. How do you optimize a slow SQL query?

Answer:

• Use indexes
• Avoid SELECT *
• Use JOIN properly
• Avoid unnecessary subqueries
• Analyze with EXPLAIN or QUERY PLAN

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Top 20 SQL Interview Questions and Answers