🚀 Data Analytics Interview Questions & Answers – SQL (Part 1) 📊🔥1. What is SQL?Answer:SQL (Structured Query Language) is used to communicate with relational databases. It helps retrieve, insert, update, and delete data.
SELECT * FROM Employees;
2. What is the difference between SQL and MySQL?SQL : A language
MySQL : A database system
SQL : Used to write queries
MySQL : Executes SQL queries
SQL : Standard language
MySQL : Software product
3. What are Primary Keys and Foreign Keys? Primary Key: Uniquely identifies each row in a table.
Foreign Key: Creates a relationship between two tables.
Example: • EmployeeID → Primary Key
• DepartmentID → Foreign Key
4. What is Normalization?Answer:Normalization organizes data into multiple related tables to reduce redundancy and improve data integrity.
Benefits:✔ Reduces duplicate data
✔ Improves consistency
✔ Saves storage
5. What is Denormalization? Answer:Denormalization combines tables to improve query performance.
Benefits:✔ Faster reporting
✔ Faster data retrieval
Drawback:❌ More redundancy
6. Difference Between WHERE and HAVING? WHERE: Filters rows before aggregation.
HAVING: Filters groups after aggregation.
SELECT Department, COUNT(*)
FROM Employees
GROUP BY Department
HAVING COUNT(*) > 10;
7. Difference Between DELETE, DROP, and TRUNCATE? DELETE: Removes selected rows.
DELETE FROM Employees
WHERE EmployeeID = 101;
TRUNCATE: Removes all rows.
TRUNCATE TABLE Employees;
DROP: Deletes entire table structure.
DROP TABLE Employees;
8. Difference Between INNER JOIN and LEFT JOIN? INNER JOIN: Returns matching records only.
LEFT JOIN: Returns all records from left table and matching records from right table.
SELECT *
FROM Employees E
LEFT JOIN Departments D
ON E.DepartmentID = D.DepartmentID;
9. What is RIGHT JOIN?Returns all rows from the right table and matching rows from the left table.
10. What is FULL OUTER JOIN?Returns all matching and non-matching rows from both tables.
11. What is SELF JOIN?A table joined with itself.
Example: Employee and Manager stored in same table.
12. What is CROSS JOIN?Returns every possible combination of rows.
If: • Table A = 5 rows
• Table B = 4 rows
Result = 20 rows 13. What are Aggregate Functions?Used to perform calculations.
Examples: COUNT(), SUM(), AVG(), MIN(), MAX()
14. Difference Between COUNT and COUNT DISTINCT? COUNT(EmployeeID): Counts all values.
COUNT(DISTINCT DepartmentID): Counts unique values only.
15. What is GROUP BY?Groups rows with similar values.
SELECT Department, COUNT(*)
FROM Employees
GROUP BY Department;
16. Difference Between GROUP BY and ORDER BY?GROUP BY: Groups data.
ORDER BY: Sorts data.
17. What is a Subquery?A query inside another query.
SELECT *
FROM Employees
WHERE Salary >
(
SELECT AVG(Salary)
FROM Employees
);
18. What are CTEs?Common Table Expressions create temporary result sets.
WITH SalesCTE AS
(
SELECT *
FROM Sales
)
SELECT *
FROM SalesCTE;
Benefits:✔ Readability
✔ Reusability
19. What are Window Functions?Perform calculations without collapsing rows.
Examples: ROW_NUMBER(), RANK(), DENSE_RANK()
20. Explain ROW_NUMBER()Assigns unique numbers.
