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1. What is SQL?

SQL (Structured Query Language) is a language used to store, manage, and retrieve data from a database.

Example:

SELECT * FROM employees;

This gets all records from the employees table.

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2. What is a Database?

A database is a place where data is stored in an organized way so it can be easily accessed.

Example:
MySQL database is like a folder that holds tables.

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3. What are the types of SQL commands?

1. DDL – Data Definition Language (CREATE, ALTER, DROP)
2. DML – Data Manipulation Language (INSERT, UPDATE, DELETE)
3. DQL – Data Query Language (SELECT)
4. TCL – Transaction Control (COMMIT, ROLLBACK)
5. DCL – Data Control (GRANT, REVOKE)

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4. What is Primary Key?

A Primary Key uniquely identifies each row in a table.

• No duplicates
• Cannot be NULL

Example:

CREATE TABLE employees (
   emp_id INT PRIMARY KEY,
   name VARCHAR(50)
);

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5. What is Foreign Key?

A Foreign Key links two tables together.
It refers to the primary key of another table.

Example:

CREATE TABLE orders (
   order_id INT PRIMARY KEY,
   emp_id INT,
   FOREIGN KEY (emp_id) REFERENCES employees(emp_id)
);

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6. What is UNIQUE Key?

UNIQUE ensures all values in a column are different.
But NULL is allowed (MySQL allows multiple NULLs).

Example:

ALTER TABLE employees ADD UNIQUE (email);

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7. Difference between Primary Key and UNIQUE Key?

Primary Key	UNIQUE Key
Only one per table	Many UNIQUE allowed
No NULL allowed	NULL allowed
Identifies a record	Ensures uniqueness only

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8. What is NOT NULL constraint?

NOT NULL ensures a column must always have a value.

Example:

name VARCHAR(50) NOT NULL

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9. What is Default Constraint?

It gives a column a default value if no value is provided.

Example:

ALTER TABLE employees 
ADD COLUMN status VARCHAR(10) DEFAULT 'Active';

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10. Difference between DELETE, TRUNCATE, and DROP?

Command	Meaning	Rollback	Removes Structure?
DELETE	Removes rows	Yes	No
TRUNCATE	Removes all rows faster	No	No
DROP	Removes entire table	No	Yes

Example:

DELETE FROM employees WHERE emp_id=1;
TRUNCATE TABLE employees;
DROP TABLE employees;

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11. Difference between WHERE and HAVING?

WHERE	HAVING
Filters rows before grouping	Filters groups after grouping
Cannot use aggregate (SUM, COUNT)	Can use aggregate

Example:

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

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12. What are Joins in SQL?

Joins combine data from two or more tables using related columns.

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13. What is INNER JOIN?

Returns matching records from both tables.

Example:

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

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14. What is LEFT JOIN?

Returns all records from the left table + matching records from the right.

Example:

SELECT e.name, d.dept_name
FROM employees e
LEFT JOIN departments d
ON e.dept_id = d.dept_id;

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15. What is RIGHT JOIN?

Returns all records from the right table + matching records from the left.

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16. What is FULL JOIN?

MySQL does NOT directly support FULL JOIN, but you can simulate it:

SELECT * FROM employees e
LEFT JOIN departments d ON e.dept_id=d.dept_id
UNION
SELECT * FROM employees e
RIGHT JOIN departments d ON e.dept_id=d.dept_id;

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17. What is Self Join?

A table joins with itself.

Example:
Find employees and their managers (same table):

SELECT e.name AS Employee, m.name AS Manager
FROM employees e
LEFT JOIN employees m
ON e.manager_id = m.emp_id;

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18. What is Cross Join?

Returns all combinations of both tables (Cartesian product).

SELECT *
FROM employees
CROSS JOIN departments;

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19. What is UNION and UNION ALL?

• UNION → Removes duplicates
• UNION ALL → Keeps duplicates

Example:

SELECT name FROM employees
UNION
SELECT name FROM managers;

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20. Difference between UNION and UNION ALL?

UNION	UNION ALL
Removes duplicates	Keeps duplicates
Slower	Faster

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21. What is Normalization?

A process of organizing data to reduce redundancy and improve integrity.

Example: splitting repeated data into separate tables.

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22. What is Denormalization?

Adding redundant data to improve read performance.

Example: storing total salary in a summary table.

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23. Difference between CHAR and VARCHAR?

CHAR	VARCHAR
Fixed length	Variable length
Faster	Saves space
Pads with spaces	No padding

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24. Difference between SQL and MySQL?

• SQL → Language
• MySQL → Database management system that uses SQL

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25. What is Auto Increment in SQL?

Automatically generates a unique number for each new row.

Example:

CREATE TABLE employees (
  emp_id INT AUTO_INCREMENT PRIMARY KEY,
  name VARCHAR(50)
);