SQL Programming Resources

𝗜𝗻𝘁𝗲𝗿𝘃𝗶𝗲𝘄𝗲𝗿: You have 2 minutes to solve this SQL query.
Retrieve the department name and the highest salary in each department from the employees table, but only for departments where the highest salary is greater than $70,000.

𝗠𝗲: Challenge accepted!

SELECT department, MAX(salary) AS highest_salary
FROM employees
GROUP BY department
HAVING MAX(salary) > 70000;

I used GROUP BY to group employees by department, MAX() to get the highest salary, and HAVING to filter the result based on the condition that the highest salary exceeds $70,000. This solution effectively shows my understanding of aggregation functions and how to apply conditions on the result of those aggregations.

𝗧𝗶𝗽 𝗳𝗼𝗿 𝗦𝗤𝗟 𝗝𝗼𝗯 𝗦𝗲𝗲𝗸𝗲𝗿𝘀:
It's not about writing complex queries; it's about writing clean, efficient, and scalable code. Focus on mastering subqueries, joins, and aggregation functions to stand out!

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✅ Core SQL Interview Questions With Answers 🖥️

1 What is SQL
• SQL stands for Structured Query Language
• You use it to read and manage data in relational databases
• Used in MySQL, PostgreSQL, SQL Server, Oracle

2 What is an RDBMS
• Relational Database Management System
• Stores data in tables with rows and columns
• Uses keys to link tables
• Example. Customer table linked to Orders table using customer_id

3 What is a table
• Structured storage for data
• Rows are records
• Columns are attributes
• Example. One row equals one customer

4 What is a primary key
• Uniquely identifies each row
• Cannot be NULL
• No duplicate values
• Example. user_id in users table

5 What is a foreign key
• Links one table to another
• Refers to a primary key in another table
• Allows duplicate values
• Example. user_id in orders table

6 Difference between primary key and foreign key
• Primary key ensures uniqueness
• Foreign key ensures relationship
• One table can have one primary key
• One table can have multiple foreign keys

7 What is NULL
• Represents missing or unknown value
• Not equal to zero or empty string
• Use IS NULL or IS NOT NULL to check

8 What are constraints
• Rules applied on columns
• Maintain data quality
• Common constraints
– NOT NULL
– UNIQUE
– PRIMARY KEY
– FOREIGN KEY
– CHECK

9 What are data types
• Define type of data stored
• Common types
– INT for numbers
– VARCHAR for text
– DATE for dates
– FLOAT or DECIMAL for decimals

10 Interview tip you must remember
• Always explain with a small example
• Speak logic before syntax
• Keep answers short and direct

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🧠 SQL Interview Question (Top-N per Group + Tie Handling - Tricky)
📌

scores(student_id, subject, marks)

❓ Ques :

👉 Find students who scored the highest marks in each subject.

👉 If multiple students have the same top score, include all of them (handle ties).

🧩 How Interviewers Expect You to Think

• Group data by subject 📚
• Identify highest marks within each group
• Handle ties correctly (don’t lose rows)
• Use window functions (RANK vs ROW_NUMBER)

💡 SQL Solution

SELECT
student_id,
subject,
marks
FROM (
SELECT
scores.student_id,
scores.subject,
scores.marks,
RANK() OVER (
PARTITION BY scores.subject
ORDER BY scores.marks DESC
) AS rnk
FROM scores
) ranked_scores
WHERE ranked_scores.rnk = 1;

🔥 Why This Question Is Powerful

• Tests Top-N per group (very common pattern) 🧠
• Checks understanding of RANK vs ROW_NUMBER

❤️ React if you want more real interview-level SQL questions 🚀

✅ Basic SQL Queries Interview Questions With Answers 🖥️

1. What does SELECT do
– SELECT fetches data from a table
– You choose columns you want to see
Example: SELECT name, salary FROM employees;

2. What does FROM do
– FROM tells SQL where data lives
– It specifies the table name
Example: SELECT * FROM customers;

3. What is WHERE clause
– WHERE filters rows
– It runs before aggregation
Example: SELECT * FROM orders WHERE status = 'Delivered';

4. Difference between WHERE and HAVING
– WHERE filters rows before GROUP BY
– HAVING filters groups after aggregation
Example: WHERE filters orders, HAVING filters total_sales

5. How do you sort data
– Use ORDER BY
– Default order is ASC
Example: SELECT * FROM employees ORDER BY salary DESC;

6. How do you sort by multiple columns
– SQL sorts left to right
Example: SELECT * FROM students ORDER BY class ASC, marks DESC;

7. What is LIMIT
– LIMIT restricts number of rows returned
– Useful for top N queries
Example: SELECT * FROM products LIMIT 5;

8. What is OFFSET
– OFFSET skips rows
– Used with LIMIT for pagination
Example: SELECT * FROM products LIMIT 5 OFFSET 10;

9. How do you filter on multiple conditions
– Use AND, OR
Example: SELECT * FROM users WHERE city = 'Delhi' AND age > 25;

10. Difference between AND and OR
– AND needs all conditions true
– OR needs one condition true

Quick interview advice
• Always say execution order: FROM → WHERE → SELECT → ORDER BY → LIMIT
• Write clean examples
• Speak logic first, syntax next¹

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Data Analyst INTERVIEW QUESTIONS AND ANSWERS
👇👇

1.Can you name the wildcards in Excel?

Ans: There are 3 wildcards in Excel that can ve used in formulas.

Asterisk (*) – 0 or more characters. For example, Ex* could mean Excel, Extra, Expertise, etc.

Question mark (?) – Represents any 1 character. For example, R?ain may mean Rain or Ruin.

Tilde (~) – Used to identify a wildcard character (~, *, ?). For example, If you need to find the exact phrase India* in a list. If you use India* as the search string, you may get any word with India at the beginning followed by different characters (such as Indian, Indiana). If you have to look for India” exclusively, use ~.

Hence, the search string will be india~*. ~ is used to ensure that the spreadsheet reads the following character as is, and not as a wildcard.


2.What is cascading filter in tableau?

Ans: Cascading filters can also be understood as giving preference to a particular filter and then applying other filters on previously filtered data source. Right-click on the filter you want to use as a main filter and make sure it is set as all values in dashboard then select the subsequent filter and select only relevant values to cascade the filters. This will improve the performance of the dashboard as you have decreased the time wasted in running all the filters over complete data source.


3.What is the difference between .twb and .twbx extension?

Ans:
A .twb file contains information on all the sheets, dashboards and stories, but it won’t contain any information regarding data source. Whereas .twbx file contains all the sheets, dashboards, stories and also compressed data sources. For saving a .twbx extract needs to be performed on the data source. If we forward .twb file to someone else than they will be able to see the worksheets and dashboards but won’t be able to look into the dataset.


4.What are the various Power BI versions?

Power BI Premium capacity-based license, for example, allows users with a free license to act on content in workspaces with Premium capacity. A user with a free license can only use the Power BI service to connect to data and produce reports and dashboards in My Workspace outside of Premium capacity. They are unable to exchange material or publish it in other workspaces. To process material, a Power BI license with a free or Pro per-user license only uses a shared and restricted capacity. Users with a Power BI Pro license can only work with other Power BI Pro users if the material is stored in that shared capacity. They may consume user-generated information, post material to app workspaces, share dashboards, and subscribe to dashboards and reports. Pro users can share material with users who don’t have a Power BI Pro subscription while workspaces are at Premium capacity.

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SQL From Basic to Advanced level

Basic SQL is ONLY 7 commands:
- SELECT
- FROM
- WHERE (also use SQL comparison operators such as =, <=, >=, <> etc.)
- ORDER BY
- Aggregate functions such as SUM, AVERAGE, COUNT etc.
- GROUP BY
- CREATE, INSERT, DELETE, etc.
You can do all this in just one morning.

Once you know these, take the next step and learn commands like:
- LEFT JOIN
- INNER JOIN
- LIKE
- IN
- CASE WHEN
- HAVING (undertstand how it's different from GROUP BY)
- UNION ALL
This should take another day.

Once both basic and intermediate are done, start learning more advanced SQL concepts such as:
- Subqueries (when to use subqueries vs CTE?)
- CTEs (WITH AS)
- Stored Procedures
- Triggers
- Window functions (LEAD, LAG, PARTITION BY, RANK, DENSE RANK)
These can be done in a couple of days.
Learning these concepts is NOT hard at all

- what takes time is practice and knowing what command to use when. How do you master that?
- First, create a basic SQL project
- Then, work on an intermediate SQL project (search online) -

Lastly, create something advanced on SQL with many CTEs, subqueries, stored procedures and triggers etc.

This is ALL you need to become a badass in SQL, and trust me when I say this, it is not rocket science. It's just logic.

Remember that practice is the key here. It will be more clear and perfect with the continous practice

Best telegram channel to learn SQL: https://t.me/sqlanalyst

Data Analyst Jobs👇
https://t.me/jobs_SQL

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🔥 Top SQL Projects for Data Analytics 🚀

If you're preparing for a Data Analyst role or looking to level up your SQL skills, working on real-world projects is the best way to learn!

Here are some must-do SQL projects to strengthen your portfolio. 👇

🟢 Beginner-Friendly SQL Projects (Great for Learning Basics)

✅ Employee Database Management – Build and query HR data 📊
✅ Library Book Tracking – Create a database for book loans and returns
✅ Student Grading System – Analyze student performance data
✅ Retail Point-of-Sale System – Work with sales and transactions 💰
✅ Hotel Booking System – Manage customer bookings and check-ins 🏨

🟡 Intermediate SQL Projects (For Stronger Querying & Analysis)

⚡ E-commerce Order Management – Analyze order trends & customer data 🛒
⚡ Sales Performance Analysis – Work with revenue, profit margins & KPIs 📈
⚡ Inventory Control System – Optimize stock tracking 📦
⚡ Real Estate Listings – Manage and analyze property data 🏡
⚡ Movie Rating System – Analyze user reviews & trends 🎬

🔵 Advanced SQL Projects (For Business-Level Analytics)

🔹 Social Media Analytics – Track user engagement & content trends
🔹 Insurance Claim Management – Fraud detection & risk assessment
🔹 Customer Feedback Analysis – Perform sentiment analysis on reviews ⭐
🔹 Freelance Job Platform – Match freelancers with project opportunities
🔹 Pharmacy Inventory System – Optimize stock levels & prescriptions

🔴 Expert-Level SQL Projects (For Data-Driven Decision Making)

🔥 Music Streaming Analysis – Study user behavior & song trends 🎶
🔥 Healthcare Prescription Tracking – Identify patterns in medicine usage
🔥 Employee Shift Scheduling – Optimize workforce efficiency ⏳
🔥 Warehouse Stock Control – Manage supply chain data efficiently
🔥 Online Auction System – Analyze bidding patterns & sales performance 🛍️

🔗 Pro Tip: If you're applying for Data Analyst roles, pick 3-4 projects, clean the data, and create interactive dashboards using Power BI/Tableau to showcase insights!

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Quick recap of essential SQL basics 😄👇

SQL is a domain-specific language used for managing and querying relational databases. It's crucial for interacting with databases, retrieving, storing, updating, and deleting data. Here are some fundamental SQL concepts:

1. Database
- A database is a structured collection of data. It's organized into tables, and SQL is used to manage these tables.

2. Table
- Tables are the core of a database. They consist of rows and columns, and each row represents a record, while each column represents a data attribute.

3. Query
- A query is a request for data from a database. SQL queries are used to retrieve information from tables. The SELECT statement is commonly used for this purpose.

4. Data Types
- SQL supports various data types (e.g., INTEGER, TEXT, DATE) to specify the kind of data that can be stored in a column.

5. Primary Key
- A primary key is a unique identifier for each row in a table. It ensures that each row is distinct and can be used to establish relationships between tables.

6. Foreign Key
- A foreign key is a column in one table that links to the primary key in another table. It creates relationships between tables in a database.

7. CRUD Operations
- SQL provides four primary operations for data manipulation:
- Create (INSERT) - Add new records to a table.
- Read (SELECT) - Retrieve data from one or more tables.
- Update (UPDATE) - Modify existing data.
- Delete (DELETE) - Remove records from a table.

8. WHERE Clause
- The WHERE clause is used in SELECT, UPDATE, and DELETE statements to filter and conditionally manipulate data.

9. JOIN
- JOIN operations are used to combine data from two or more tables based on a related column. Common types include INNER JOIN, LEFT JOIN, and RIGHT JOIN.

10. Index
- An index is a database structure that improves the speed of data retrieval operations. It's created on one or more columns in a table.

11. Aggregate Functions
- SQL provides functions like SUM, AVG, COUNT, MAX, and MIN for performing calculations on groups of data.

12. Transactions
- Transactions are sequences of one or more SQL statements treated as a single unit. They ensure data consistency by either applying all changes or none.

13. Normalization
- Normalization is the process of organizing data in a database to minimize data redundancy and improve data integrity.

14. Constraints
- Constraints (e.g., NOT NULL, UNIQUE, CHECK) are rules that define what data is allowed in a table, ensuring data quality and consistency.

Here is an amazing resources to learn & practice SQL: https://bit.ly/3FxxKPz

Share with credits: https://t.me/sqlspecialist

Hope it helps :)

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Don’t miss this power-packed masterclass by Tushar Jha, Lead Data Scientist at Google.

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SQL Cheat Sheet for Data Analysts 🗄️📊

1. SELECT
What it is: Used to choose columns from a table
What it does: Returns specific columns of data

Query: Fetch name and salary

SELECT name, salary 
FROM employees;

2. FROM
What it is: Specifies the table
What it does: Tells SQL where to get data from

Query: Fetch all data from employees

SELECT * 
FROM employees;

3. WHERE
What it is: Filters rows based on condition
What it does: Returns only matching rows

Query: Employees with salary > 30000

SELECT * 
FROM employees 
WHERE salary > 30000;

4. ORDER BY
What it is: Sorts the data
What it does: Arranges rows in order

Query: Sort by salary (highest first)

SELECT * 
FROM employees 
ORDER BY salary DESC;

5. COUNT()
What it is: Counts rows
What it does: Returns total records

Query: Count employees

SELECT COUNT(*) 
FROM employees;

6. AVG()
What it is: Calculates average
What it does: Returns mean value

Query: Average salary

SELECT AVG(salary) 
FROM employees;

7. GROUP BY
What it is: Groups rows by column
What it does: Applies aggregation per group

Query: Avg salary per department

SELECT department, AVG(salary) 
FROM employees 
GROUP BY department;

8. HAVING
What it is: Filters grouped data
What it does: Returns filtered groups

Query: Departments with avg salary > 40000

SELECT department, AVG(salary) 
FROM employees 
GROUP BY department 
HAVING AVG(salary) > 40000;

9. INNER JOIN
What it is: Combines matching rows from tables
What it does: Returns common data

Query: Employees with department names

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

10. LEFT JOIN
What it is: Combines all left + matching right
What it does: Returns all left table data

Query: All employees with departments

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

11. CASE WHEN
What it is: Conditional logic
What it does: Creates values based on condition

Query: Categorize salary

SELECT name, 
    CASE 
        WHEN salary > 40000 THEN 'High' 
        ELSE 'Low' 
    END AS category 
FROM employees;

12. SUBQUERY
What it is: Query inside another query
What it does: Uses result of inner query

Query: Salary above average

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

13. RANK()
What it is: Window function
What it does: Assigns rank without grouping

Query: Rank employees by salary

SELECT name, salary, 
    RANK() OVER (ORDER BY salary DESC) AS rank 
FROM employees;

14. DISTINCT
What it is: Removes duplicates
What it does: Returns unique values

Query: Unique departments

SELECT DISTINCT department 
FROM employees;

15. LIKE
What it is: Pattern matching
What it does: Filters text patterns

Query: Names starting with A

SELECT * 
FROM employees 
WHERE name LIKE 'A%';

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📊 Complete SQL Syllabus Roadmap (Beginner to Expert) 🗄️

🔰 Beginner Level:

1. Intro to Databases: What are databases, Relational vs. Non-Relational
2. SQL Basics: SELECT, FROM, WHERE
3. Data Types: INT, VARCHAR, DATE, BOOLEAN, etc.
4. Operators: Comparison, Logical (AND, OR, NOT)
5. Sorting & Filtering: ORDER BY, LIMIT, DISTINCT
6. Aggregate Functions: COUNT, SUM, AVG, MIN, MAX
7. GROUP BY and HAVING: Grouping Data and Filtering Groups
8. Basic Projects: Creating and querying a simple database (e.g., a student database)

⚙️ Intermediate Level:

1. Joins: INNER, LEFT, RIGHT, FULL OUTER JOIN
2. Subqueries: Using queries within queries
3. Indexes: Improving Query Performance
4. Data Modification: INSERT, UPDATE, DELETE
5. Transactions: ACID Properties, COMMIT, ROLLBACK
6. Constraints: PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL, CHECK, DEFAULT
7. Views: Creating Virtual Tables
8. Stored Procedures & Functions: Reusable SQL Code
9. Date and Time Functions: Working with Date and Time Data
10. Intermediate Projects: Designing and querying a more complex database (e.g., an e-commerce database)

🏆 Expert Level:

1. Window Functions: RANK, ROW_NUMBER, LAG, LEAD
2. Common Table Expressions (CTEs): Recursive and Non-Recursive
3. Performance Tuning: Query Optimization Techniques
4. Database Design & Normalization: Understanding Database Schemas (Star, Snowflake)
5. Advanced Indexing: Clustered, Non-Clustered, Filtered Indexes
6. Database Administration: Backup and Recovery, Security, User Management
7. Working with Large Datasets: Partitioning, Data Warehousing Concepts
8. NoSQL Databases: Introduction to MongoDB, Cassandra, etc. (optional)
9. SQL Injection Prevention: Secure Coding Practices
10. Expert Projects: Designing, optimizing, and managing a large-scale database (e.g., a social media database)

💡 Bonus: Learn about Database Security, Cloud Databases (AWS RDS, Azure SQL Database, Google Cloud SQL), and Data Modeling Tools.

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🔥 Top SQL Interview Questions with Answers

🎯 1️⃣ Find 2nd Highest Salary
📊 Table: employees
id | name | salary
1 | Rahul | 50000
2 | Priya | 70000
3 | Amit | 60000
4 | Neha | 70000

❓ Problem Statement: Find the second highest distinct salary from the employees table.

✅ Solution
SELECT MAX(salary) FROM employees WHERE salary < ( SELECT MAX(salary) FROM employees );

🎯 2️⃣ Find Nth Highest Salary
📊 Table: employees
id | name | salary
1 | A | 100
2 | B | 200
3 | C | 300
4 | D | 200

❓ Problem Statement: Write a query to find the 3rd highest salary.

✅ Solution
SELECT salary FROM ( SELECT salary, DENSE_RANK() OVER(ORDER BY salary DESC) r FROM employees ) t WHERE r = 3;

🎯 3️⃣ Find Duplicate Records
📊 Table: employees
id | name
1 | Rahul
2 | Amit
3 | Rahul
4 | Neha

❓ Problem Statement: Find all duplicate names in the employees table.

✅ Solution
SELECT name, COUNT(*) FROM employees GROUP BY name HAVING COUNT(*) > 1;

🎯 4️⃣ Customers with No Orders
📊 Table: customers
customer_id | name
1 | Rahul
2 | Priya
3 | Amit

📊 Table: orders
order_id | customer_id
101 | 1
102 | 2

❓ Problem Statement: Find customers who have not placed any orders.

✅ Solution
SELECT c.name FROM customers c LEFT JOIN orders o ON c.customer_id = o.customer_id WHERE o.customer_id IS NULL;

🎯 5️⃣ Top 3 Salaries per Department
📊 Table: employees
name | department | salary
A | IT | 100
B | IT | 200
C | IT | 150
D | HR | 120
E | HR | 180

❓ Problem Statement: Find the top 3 highest salaries in each department.

✅ Solution
SELECT * FROM ( SELECT name, department, salary, ROW_NUMBER() OVER( PARTITION BY department ORDER BY salary DESC ) r FROM employees ) t WHERE r <= 3;

🎯 6️⃣ Running Total of Sales
📊 Table: sales
date | sales
2024-01-01 | 100
2024-01-02 | 200
2024-01-03 | 300

❓ Problem Statement: Calculate the running total of sales by date.

✅ Solution
SELECT date, sales, SUM(sales) OVER(ORDER BY date) AS running_total FROM sales;

🎯 7️⃣ Employees Above Average Salary
📊 Table: employees
name | salary
A | 100
B | 200
C | 300

❓ Problem Statement: Find employees earning more than the average salary.

✅ Solution
SELECT name, salary FROM employees WHERE salary > ( SELECT AVG(salary) FROM employees );

🎯 8️⃣ Department with Highest Total Salary
📊 Table: employees
name | department | salary
A | IT | 100
B | IT | 200
C | HR | 500

❓ Problem Statement: Find the department with the highest total salary.

✅ Solution
SELECT department, SUM(salary) AS total_salary FROM employees GROUP BY department ORDER BY total_salary DESC LIMIT 1;

🎯 9️⃣ Customers Who Placed Orders
📊 Tables: Same as Q4
❓ Problem Statement: Find customers who have placed at least one order.

✅ Solution
SELECT name FROM customers c WHERE EXISTS ( SELECT 1 FROM orders o WHERE c.customer_id = o.customer_id );

🎯 🔟 Remove Duplicate Records
📊 Table: employees
id | name
1 | Rahul
2 | Rahul
3 | Amit

❓ Problem Statement: Delete duplicate records but keep one unique record.

✅ Solution
DELETE FROM employees WHERE id NOT IN ( SELECT MIN(id) FROM employees GROUP BY name );

🚀 Pro Tip:
👉 In interviews:
First explain logic
Then write query
Then optimize

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⚙️ SQL Developer Roadmap

📂 SQL Basics (CREATE, DROP, USE Database)
∟📂 Data Types & DDL (Tables, Constraints - PK/FK)
∟📂 DML (INSERT, UPDATE, DELETE)
∟📂 SELECT Queries (DISTINCT, LIMIT/TOP)
∟📂 WHERE Clause (Operators, LIKE, IN, BETWEEN)
∟📂 ORDER BY & Sorting (ASC/DESC)
∟📂 Aggregate Functions (COUNT, SUM, AVG, MIN/MAX)
∟📂 GROUP BY & HAVING
∟📂 JOINs (INNER, LEFT, RIGHT, FULL)
∟📂 Subqueries
∟📂 String Functions (CONCAT, SUBSTRING, UPPER/LOWER)
∟📂 Date Functions (NOW, DATEADD, DATEDIFF)
∟📂 Window Functions (ROW_NUMBER, RANK, PARTITION BY)
∟📂 CTEs (Common Table Expressions)
∟📂 Indexes & Performance
∟📂 Transactions (BEGIN, COMMIT, ROLLBACK)
∟📂 Views & Stored Procedures
∟📂 Practice (LeetCode SQL, HackerRank)
∟✅ Apply for Data Analyst / Backend Roles

💬 Tap ❤️ for more!

✅ Useful Platform to Practice SQL Programming 🧠🖥️

Learning SQL is just the first step — practice is what builds real skill. Here are the best platforms for hands-on SQL:

1️⃣ LeetCode – For Interview-Oriented SQL Practice
• Focus: Real interview-style problems
• Levels: Easy to Hard
• Schema + Sample Data Provided
• Great for: Data Analyst, Data Engineer, FAANG roles
✔ Tip: Start with Easy → filter by “Database” tag
✔ Popular Section: Database → Top 50 SQL Questions
Example Problem: “Find duplicate emails in a user table” → Practice filtering, GROUP BY, HAVING

2️⃣ HackerRank – Structured & Beginner-Friendly
• Focus: Step-by-step SQL track
• Has certification tests (SQL Basic, Intermediate)
• Problem sets by topic: SELECT, JOINs, Aggregations, etc.
✔ Tip: Follow the full SQL track
✔ Bonus: Company-specific challenges
Try: “Revising Aggregations – The Count Function” → Build confidence with small wins

3️⃣ Mode Analytics – Real-World SQL in Business Context
• Focus: Business intelligence + SQL
• Uses real-world datasets (e.g., e-commerce, finance)
• Has an in-browser SQL editor with live data
✔ Best for: Practicing dashboard-level queries
✔ Tip: Try the SQL case studies & tutorials

4️⃣ StrataScratch – Interview Questions from Real Companies
• 500+ problems from companies like Uber, Netflix, Google
• Split by company, difficulty, and topic
✔ Best for: Intermediate to advanced level
✔ Tip: Try “Hard” questions after doing 30–50 easy/medium

5️⃣ DataLemur – Short, Practical SQL Problems
• Crisp and to the point
• Good UI, fast learning
• Real interview-style logic
✔ Use when: You want fast, smart SQL drills

📌 How to Practice Effectively:
• Spend 20–30 mins/day
• Focus on JOINs, GROUP BY, HAVING, Subqueries
• Analyze problem → write → debug → re-write
• After solving, explain your logic out loud

🧪 Practice Task:
Try solving 5 SQL questions from LeetCode or HackerRank this week. Start with SELECT, WHERE, and GROUP BY.

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🚀 Window Functions ⭐

Window functions are one of the most powerful SQL features used in data analytics, reporting, and advanced SQL interviews.

👉 They allow you to perform calculations across rows without collapsing them like GROUP BY.

In simple words:
GROUP BY → reduces rows

Window Functions → keep rows but add calculated values

🧠 Basic Syntax of Window Functions

SELECT column, window_function() 
OVER ( 
  PARTITION BY column 
  ORDER BY column 
) 
FROM table;

Components:
- OVER() → defines the window
- PARTITION BY → splits data into groups
- ORDER BY → defines calculation order

🔹 1️⃣ ROW_NUMBER()

Assigns a unique sequential number to rows.

SELECT name, salary, ROW_NUMBER() OVER(ORDER BY salary DESC) AS rank 
FROM employees;

Result:
name | salary | rank
Rahul | 90000 | 1
Priya | 85000 | 2
Amit | 85000 | 3

👉 Even if salaries are same, numbers stay unique.

🔹 2️⃣ RANK()

Assigns rank but skips numbers when ties occur.

SELECT name, salary, RANK() OVER(ORDER BY salary DESC) AS rank 
FROM employees;

Result:
name | salary | rank
Rahul | 90000 | 1
Priya | 85000 | 2
Amit | 85000 | 2
Neha | 80000 | 4

Notice rank 3 is skipped.

🔹 3️⃣ DENSE_RANK()

Similar to RANK but does not skip numbers.

SELECT name, salary, DENSE_RANK() OVER(ORDER BY salary DESC) AS rank 
FROM employees;

Result:
name | salary | rank
Rahul | 90000 | 1
Priya | 85000 | 2
Amit | 85000 | 2
Neha | 80000 | 3

🔹 4️⃣ PARTITION BY

Used to divide rows into groups before calculation.

Example: Rank employees within each department

SELECT name, department, salary, 
RANK() OVER( 
  PARTITION BY department 
  ORDER BY salary DESC 
) AS dept_rank 
FROM employees;

👉 Each department gets its own ranking.

🔹 5️⃣ LAG()

Used to access previous row values.

Example: Compare sales with previous day.

SELECT date, sales, LAG(sales) OVER(ORDER BY date) AS previous_sales 
FROM sales;

🔹 6️⃣ LEAD()

Used to access next row values.

SELECT date, sales, LEAD(sales) OVER(ORDER BY date) AS next_sales 
FROM sales;

⭐ Real Data Analyst Examples

Top 3 highest salaries

SELECT ** 
FROM ( 
  SELECT name, salary, ROW_NUMBER() OVER(ORDER BY salary DESC) AS rn 
  FROM employees 
) t 
WHERE rn <= 3;

Running total of sales

SELECT date, sales, SUM(sales) OVER(ORDER BY date) AS running_total 
FROM sales;

Rank products by category

SELECT product_name, category, price, 
RANK() OVER(PARTITION BY category ORDER BY price DESC) AS rank 
FROM products;

🎯 Common Interview Questions
✔ Difference between ROW_NUMBER, RANK, DENSE_RANK
✔ Find Nth highest salary
✔ Running totals using window functions
✔ Compare current row with previous row
✔ Rank employees by department

🚀 Mini Practice Tasks
Task 1: Assign row numbers to employees by salary.
Task 2: Rank employees by salary.
Task 3: Find top 3 highest salaries using window functions.
Task 4: Calculate running total of sales.

💼 What You Must Master
✅ ROW_NUMBER()
✅ RANK()
✅ DENSE_RANK()
✅ PARTITION BY
✅ LAG() / LEAD()
✅ Running totals

These functions are used heavily in real analytics queries and SQL interviews.

Double Tap ♥️ For More

🧠 SQL Interview Question (Moderate–Tricky & Duplicate Transaction Detection)
📌

transactions(transaction_id, user_id, transaction_date, amount)

❓ Ques :

👉 Find users who made multiple transactions with the same amount consecutively.

🧩 How Interviewers Expect You to Think

• Sort transactions chronologically for each user
• Compare the current transaction amount with the previous one
• Use a window function to detect consecutive duplicates

💡 SQL Solution

SELECT
user_id,
transaction_date,
amount
FROM (
SELECT
user_id,
transaction_date,
amount,
LAG(amount) OVER (
PARTITION BY user_id
ORDER BY transaction_date
) AS prev_amount
FROM transactions
) t
WHERE amount = prev_amount;

🔥 Why This Question Is Powerful

• Tests understanding of LAG() for row comparison
• Evaluates ability to identify patterns in sequential data
• Reflects real-world use cases like detecting suspicious or duplicate transactions

❤️ React if you want more tricky real interview-level SQL questions 🚀