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🔰 Introduction to Databases & SQL

What is a Database?
A database is an organized collection of data that allows for easy access, management, and updating. Think of it like a digital filing system.

Types of Databases:

1. Relational Databases – Store data in tables (like Excel). Examples: MySQL, PostgreSQL, SQL Server.


2. Non-Relational (NoSQL) – Store data as documents, key-value pairs, etc. Examples: MongoDB, Redis.



What is SQL?
Structured Query Language (SQL) is the standard language used to communicate with relational databases. It allows you to create, read, update, and delete data — often remembered by the acronym CRUD.

Why Learn SQL?

SQL is foundational for data analysis, data science, backend development, and database administration.

It’s used across industries to manage and analyze large volumes of data.


Real-World Example:
Imagine you're a data analyst at a retail company. SQL helps you answer questions like:

"How many orders were placed in the last 30 days?"

"What’s the average purchase value by city?"

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📄 SQL vs NoSQL

1. What is SQL (Relational) Database?
SQL databases are structured and use tables (rows and columns) to store data. They follow a strict schema, meaning the data format is predefined.

Examples: MySQL, PostgreSQL, SQLite, SQL Server

Used For: Applications where data integrity and relationships are important, like banking systems or e-commerce platforms.

2. What is NoSQL (Non-Relational) Database?

NoSQL databases are more flexible and can store unstructured or semi-structured data like JSON or key-value pairs. They don’t require a fixed schema.

Examples: MongoDB, Redis, Firebase, Cassandra

Used For: Real-time applications, large-scale data, or when rapid development and scalability are more important than structure.

Key Differences:

Data Format: SQL uses tables; NoSQL uses documents or key-value pairs.
Schema: SQL is strict; NoSQL is flexible.
Scalability: SQL scales vertically (strong server); NoSQL scales horizontally (more servers).
Use Case: SQL is great for complex queries and transactions; NoSQL excels in high-volume, real-time scenarios.

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Awesome! Let’s dive into the next topic:

🧱 Database Concepts (Tables, Rows, Columns, Keys)

1. Table:
A table is the basic structure where data is stored in a relational database. Think of it like a spreadsheet. Each table represents one type of entity — for example, a Customers table or a Products table.

2. Rows (Records):
Each row in a table represents a single record or entry.
Example: A row in the Customers table could represent one customer’s details like their name, email, and phone number.

3. Columns (Fields):
Columns represent the attributes or properties of the data.

Example: In a Products table, columns might be product_id, product_name, price, and category.

4. Keys:

Keys are special columns that help in uniquely identifying rows and establishing relationships between tables.

Primary Key (PK): Uniquely identifies each record in a table. It must be unique and not null.

Example: customer_id in a Customers table.

Foreign Key (FK): A field in one table that refers to the primary key in another table. It’s used to link tables together.

Example: customer_id in an Orders table links to the Customers table.

Real-World Analogy:

Imagine a school:

The "Student" table holds data about each student.
Each row is one student.
Each column is an attribute like name, roll number, or class.

The primary key might be roll_number.

A foreign key might be class_id that links to a Classes table.

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Next up: 🔍 Basic SQL Queries (SELECT, WHERE).

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Moving on to next topic!

🔍 Basic SQL Queries (SELECT, WHERE)

1. SELECT Statement:
The SELECT command is used to retrieve data from a table. It’s the most fundamental query in SQL.

Syntax:

SELECT column1, column2 FROM table_name;

Example:

SELECT name, email FROM customers;

This fetches the name and email of all customers from the customers table.

You can also use * to select all columns:

SELECT * FROM customers;


2. WHERE Clause:
The WHERE clause is used to filter records that meet a specific condition.

Syntax:

SELECT column1, column2 FROM table_name WHERE condition;

Example:

SELECT name FROM customers WHERE city = 'Delhi';

This returns names of all customers who are from Delhi.

Another example using numbers:

SELECT * FROM products WHERE price > 1000;

This gets all products priced above 1000.


Key Point:

SELECT fetches data

WHERE filters it based on conditions


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Let’s move on to the next topic in our SQL Roadmap!

✏️ Filtering & Sorting Data (ORDER BY, LIMIT)

1. ORDER BY Clause:
ORDER BY is used to sort the result set based on one or more columns — either in ascending or descending order.

Syntax:

SELECT column1, column2 FROM table_name ORDER BY column1 ASC|DESC;

Example:

SELECT name, salary FROM employees ORDER BY salary DESC;

This lists employees with the highest salaries at the top.

By default, it sorts in ascending (ASC) order if no direction is specified.

2. LIMIT Clause:
LIMIT is used to restrict the number of rows returned by a query. Super useful when you want just a sample or the top results.

Syntax:

SELECT * FROM table_name LIMIT number;

Example:

SELECT * FROM products LIMIT 5;

This fetches only the first 5 products.

You can also combine ORDER BY and LIMIT:

SELECT * FROM products ORDER BY price DESC LIMIT 3;

This gets the top 3 most expensive products.

Quick Recap:

Use ORDER BY to sort your data

Use LIMIT to control how many results you get

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🔢 SQL Operators (IN, BETWEEN, LIKE, AND, OR)

These operators help you build flexible and powerful conditions inside your WHERE clause.


1. IN Operator
Used to match multiple values in a column.

Example:

SELECT * FROM customers WHERE city IN ('Delhi', 'Mumbai', 'Bangalore');

This fetches customers who live in any of the three cities.


2. BETWEEN Operator
Used to filter values within a range (inclusive).

Example:

SELECT * FROM orders WHERE order_date BETWEEN '2024-01-01' AND '2024-12-31';

Returns all orders placed in 2024.


3. LIKE Operator
Used for pattern matching. Especially useful with wildcards (%).

Example:

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

Finds customers whose names start with "A".

Another example:

SELECT * FROM emails WHERE address LIKE '%@gmail.com';

Finds all Gmail users.


4. AND Operator
Combines multiple conditions — all must be true.

Example:

SELECT * FROM employees WHERE department = 'HR' AND salary > 50000;

Finds HR employees earning more than 50,000.


5. OR Operator
Returns results if any one condition is true.

Example:

SELECT * FROM products WHERE category = 'Electronics' OR category = 'Books';

Fetches products that belong to either of the two categories.


Pro Tip:
Combine these operators for complex logic!

SELECT * FROM orders
WHERE status = 'Delivered'
AND delivery_date BETWEEN '2025-01-01' AND '2025-03-31';


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📊 Aggregate Functions (COUNT, SUM, AVG, MIN, MAX)

Aggregate functions are used to perform calculations on multiple rows of a table and return a single value. They're mostly used with GROUP BY, but also work standalone.

1. COUNT()
Returns the number of rows.

Example:

SELECT COUNT(*) FROM employees;

Counts all employees in the table.

You can also count only non-null values in a column:

SELECT COUNT(email) FROM customers;


2. SUM()
Adds up all the values in a numeric column.

Example:

SELECT SUM(salary) FROM employees;

Gives you the total salary payout.


3. AVG()
Calculates the average value of a numeric column.

Example:

SELECT AVG(price) FROM products;

Finds the average product price.


4. MIN()
Returns the lowest value.

Example:

SELECT MIN(salary) FROM employees;

Finds the smallest salary.


5. MAX()
Returns the highest value.

Example:

SELECT MAX(salary) FROM employees;

Finds the highest salary in the table.


Bonus Example:

SELECT
COUNT(*) AS total_orders,
SUM(amount) AS total_revenue,
AVG(amount) AS avg_order_value
FROM orders;

This gives you a quick business summary: number of orders, total revenue, and average order value.


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7 High-Impact Portfolio Project Ideas for Aspiring Data Analysts

✅ Sales Dashboard – Use Power BI or Tableau to visualize KPIs like revenue, profit, and region-wise performance
✅ Customer Churn Analysis – Predict which customers are likely to leave using Python (Logistic Regression, EDA)
✅ Netflix Dataset Exploration – Analyze trends in content types, genres, and release years with Pandas & Matplotlib
✅ HR Analytics Dashboard – Visualize attrition, department strength, and performance reviews
✅ Survey Data Analysis – Clean, visualize, and derive insights from user feedback or product surveys
✅ E-commerce Product Analysis – Analyze top-selling products, revenue by category, and return rates
✅ Airbnb Price Predictor – Use machine learning to predict listing prices based on location, amenities, and ratings

These projects showcase real-world skills and storytelling with data.

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👥 GROUP BY & HAVING Clauses

1. GROUP BY
GROUP BY is used to group rows that have the same values in specified columns and apply aggregate functions to each group.

Syntax:

SELECT column, AGG_FUNC(column2) FROM table_name GROUP BY column;

Example:

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

This shows how many employees are in each department.

You can group by multiple columns too:

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


2. HAVING
HAVING is like WHERE, but it’s used to filter grouped data. You can't use WHERE with aggregate functions — that's where HAVING comes in.

Example:

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

This gives you only those departments that have more than 5 employees.


Bonus: Combine GROUP BY + ORDER BY + HAVING:

SELECT category, SUM(sales) AS total_sales
FROM products
GROUP BY category
HAVING SUM(sales) > 10000
ORDER BY total_sales DESC;

This gives you the top-selling categories with sales over 10,000.


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🔗 SQL JOINS (INNER, LEFT, RIGHT, FULL, SELF)

JOINS help you combine data from two or more tables based on a related column (usually a primary key and a foreign key).

1. INNER JOIN
Returns only matching rows between two tables.

SELECT customers.name, orders.order_id
FROM customers
INNER JOIN orders ON customers.id = orders.customer_id;

This returns only those customers who have placed at least one order.

2. LEFT JOIN (or LEFT OUTER JOIN)
Returns all rows from the left table, and matched rows from the right table. If no match, you'll see NULLs.

SELECT customers.name, orders.order_id
FROM customers
LEFT JOIN orders ON customers.id = orders.customer_id;

This shows all customers, including those who haven’t placed any orders.

3. RIGHT JOIN (or RIGHT OUTER JOIN)
Returns all rows from the right table, and matching rows from the left.

SELECT customers.name, orders.order_id
FROM customers
RIGHT JOIN orders ON customers.id = orders.customer_id;

You’ll see all orders — even if there’s no corresponding customer info.

4. FULL JOIN (or FULL OUTER JOIN)
Returns all rows from both tables. If there's no match, it returns NULLs.

Note: MySQL doesn't support FULL JOIN directly; use UNION of LEFT and RIGHT joins instead.

5. SELF JOIN
You join a table with itself. Great for hierarchical relationships.

SELECT e.name AS employee, m.name AS manager
FROM employees e
JOIN employees m ON e.manager_id = m.id;

This shows each employee along with their manager's name.

Pro Tip: Be careful with NULLs and always define clear join conditions to avoid cartesian products.

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📦 Subqueries & Nested Queries.

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📦 Subqueries & Nested Queries

A subquery is a query inside another query. You can use it in SELECT, FROM, or WHERE clauses to solve complex problems step-by-step.

1. Subquery in WHERE Clause
Use this when you need to filter results based on another query.

SELECT name
FROM employees
WHERE department_id = (
SELECT id FROM departments WHERE name = 'Sales'
);

This finds all employees who work in the Sales department.

2. Subquery in SELECT Clause
This lets you fetch calculated or related values for each row.

SELECT name,
(SELECT AVG(salary) FROM employees) AS avg_salary
FROM employees;

Shows each employee’s name along with the company’s average salary.

3. Subquery in FROM Clause (Inline View)
Used when you want to treat the subquery like a temporary table.

SELECT department, total
FROM (
SELECT department, SUM(salary) AS total
FROM employees
GROUP BY department
) AS dept_summary;

This groups salaries by department in a subquery, then fetches from it.

Important:

- Always alias your subqueries (especially in the FROM clause).

- Avoid correlated subqueries if possible; they’re slower.


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