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๐Ÿš€ SQL Project Series

Food & Grocery Delivery Analytics ๐Ÿ›’

Analyze customers, stores, products, orders, deliveries, and payments to understand sales performance, customer behavior, delivery efficiency, and operational costs.

๐ŸŽฏ Business Objectives

โœ… Analyze order and revenue trends

โœ… Identify top-selling products

โœ… Measure customer retention

โœ… Analyze store performance

โœ… Track delivery efficiency

โœ… Identify peak ordering periods

โœ… Monitor cancellations and refunds

โœ… Optimize product and store performance

๐Ÿ“‚ Database Setup

CREATE DATABASE grocery_delivery_db;
USE grocery_delivery_db;


Tables Created:

customers โ†’ customer_id, customer_name, city, signup_date

stores โ†’ store_id, store_name, city, store_type

products โ†’ product_id, product_name, category, price

orders โ†’ order_id, customer_id, store_id, order_date, order_status, delivery_time_minutes, delivery_fee

order_items โ†’ order_item_id, order_id, product_id, quantity, unit_price

Sample data for 5 customers, 4 stores, 5 products, 5 orders already included.

๐Ÿง  SQL Concepts You'll Practice

โœ” INNER JOIN, LEFT JOIN

โœ” Aggregate Functions, GROUP BY, HAVING

โœ” CASE WHEN, CTEs, Subqueries

โœ” Window Functions

โœ” Date & Time Functions

โœ” Conditional Aggregation

๐Ÿ“Š Business KPIs You Can Build

๐Ÿ“ˆ Total Orders, Completed Orders, Cancelled Orders, Cancellation Rate

๐Ÿ“ˆ Total Revenue, AOV, Average Basket Size, Items Sold

๐Ÿ“ˆ Revenue by Category, Store, City

๐Ÿ“ˆ Top-Selling / Low-Selling Products

๐Ÿ“ˆ Customer Lifetime Value, Repeat Purchase Rate, Retention Rate

๐Ÿ“ˆ Average Delivery Time, On-Time Delivery Rate

๐Ÿ“ˆ Peak Ordering Hour, Peak Ordering Day, Monthly Revenue Growth

๐Ÿ“ˆ Delivery Fee Revenue, Customer Acquisition Trend

๐Ÿ“ˆ Executive Grocery Delivery Dashboard

๐Ÿ’ก Example Queries

1. Total Revenue

SELECT SUM(oi.quantity * oi.unit_price) AS total_revenue
FROM orders o
JOIN order_items oi ON o.order_id = oi.order_id
WHERE o.order_status = 'Delivered';


2. Top-Selling Products

SELECT p.product_name, SUM(oi.quantity) AS units_sold
FROM products p
JOIN order_items oi ON p.product_id = oi.product_id
JOIN orders o ON oi.order_id = o.order_id
WHERE o.order_status = 'Delivered'
GROUP BY p.product_name
ORDER BY units_sold DESC
LIMIT 10;


3. Average Order Value

WITH order_values AS (
SELECT o.order_id, SUM(oi.quantity * oi.unit_price) AS order_value
FROM orders o
JOIN order_items oi ON o.order_id = oi.order_id
WHERE o.order_status = 'Delivered'
GROUP BY o.order_id
)
SELECT ROUND(AVG(order_value), 2) AS average_order_value FROM order_values;


4. Repeat Customers

SELECT customer_id, COUNT(order_id) AS total_orders
FROM orders
WHERE order_status = 'Delivered'
GROUP BY customer_id
HAVING COUNT(order_id) > 1;


5. Revenue by Store

SELECT s.store_name, SUM(oi.quantity * oi.unit_price) AS revenue
FROM stores s
JOIN orders o ON s.store_id = o.store_id
JOIN order_items oi ON o.order_id = oi.order_id
WHERE o.order_status = 'Delivered'
GROUP BY s.store_name
ORDER BY revenue DESC;


6. Cancellation Rate

SELECT ROUND(100.0 * SUM(CASE WHEN order_status = 'Cancelled' THEN 1 ELSE 0 END) / COUNT(*), 2) AS cancellation_rate
FROM orders;


7. Peak Ordering Hours

SELECT EXTRACT(HOUR FROM order_date) AS order_hour, COUNT(*) AS total_orders
FROM orders
WHERE order_status = 'Delivered'
GROUP BY EXTRACT(HOUR FROM order_date)
ORDER BY total_orders DESC;


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๐Ÿ’ป HOW TO DEBUG YOUR CODE ๐Ÿ›๐Ÿ”

1๏ธโƒฃ READ THE ERROR MESSAGE

Don't ignore the error.

An error message usually tells you:

๐Ÿ‘‰ What went wrong

๐Ÿ‘‰ Where it happened

๐Ÿ‘‰ Sometimes why it happened

Example:

"NameError: name 'total' is not defined"

This tells you that Python cannot find a variable called "total".

2๏ธโƒฃ CHECK THE LINE NUMBER

Most programming errors tell you where the problem occurred.

Go directly to that line.

Then check:

โ€ข Variable names

โ€ข Syntax

โ€ข Data types

โ€ข Function calls

โ€ข Missing brackets

โ€ข Incorrect indentation

3๏ธโƒฃ UNDERSTAND THE ERROR TYPE

Common errors beginners encounter:

"SyntaxError" โ†’ Code doesn't follow the language syntax.

"NameError" โ†’ You used a name that hasn't been defined.

"TypeError" โ†’ An operation was performed on an incompatible data type.

"IndexError" โ†’ You tried to access an invalid index.

"KeyError" โ†’ A dictionary key doesn't exist.

"ValueError" โ†’ A value has the wrong format or isn't acceptable.

๐Ÿ‘‰ Learn what common errors mean instead of simply searching for fixes.

4๏ธโƒฃ CHECK YOUR ASSUMPTIONS

Sometimes your code runs without an error but produces the wrong result.

Example:

"age = "25""

You might think "age" contains a number.

But it actually contains a string.

Always ask:

๐Ÿ‘‰ What type is this variable?

๐Ÿ‘‰ What value does it currently contain?

5๏ธโƒฃ PRINT INTERMEDIATE VALUES

When you're unsure what is happening, inspect your variables.

Example:

"print(total)"

"print(count)"

"print(average)"

This helps you understand how the values change while your program runs.

6๏ธโƒฃ BREAK THE PROBLEM INTO SMALL PARTS

Don't debug 200 lines of code at once.

Separate the problem.

Instead of asking:

โŒ "Why doesn't my program work?"

Ask:

โœ… "Is my input correct?"

Then:

โœ… "Is my calculation correct?"

Then:

โœ… "Is my loop working?"

Then:

โœ… "Is my output correct?"

Small questions are easier to solve.

7๏ธโƒฃ CHECK YOUR LOOP

Loops are a common source of bugs.

Check:

๐Ÿ‘‰ Where does the loop start?

๐Ÿ‘‰ When does it stop?

๐Ÿ‘‰ Is the condition correct?

๐Ÿ‘‰ Is the variable being updated?

๐Ÿ‘‰ Could this become an infinite loop?

Example:

"while count < 10:"

" print(count)"

" count += 1"

If "count" never changes, the loop may never end.

8๏ธโƒฃ CHECK YOUR DATA TYPES

Many bugs happen because developers expect one type but receive another.

Example:

""10" + "20""

Result:

""1020""

But:

10 + 20

Result:

30

The values look similar, but their data types are different.

9๏ธโƒฃ TEST WITH SIMPLE INPUT

If your program fails with complicated data, simplify it.

Instead of:

[15, 82, 43, 91, 27, 64, 10]

Try:

[1, 2, 3]

Then:

[1]

Then:

[]

Simple inputs make problems easier to identify.

๐Ÿ”Ÿ TEST EDGE CASES

Always test unusual situations.

Examples:

โ€ข Empty input

โ€ข One element

โ€ข Duplicate values

โ€ข Negative numbers

โ€ข Very large numbers

โ€ข Missing values

โ€ข Invalid input

A solution isn't truly reliable until you understand how it behaves in these situations.

1๏ธโƒฃ1๏ธโƒฃ USE A DEBUGGER

As your programs become larger, use debugging tools.

A debugger allows you to:

๐Ÿ”น Pause execution

๐Ÿ”น Inspect variables

๐Ÿ”น Execute code step by step

๐Ÿ”น Set breakpoints

๐Ÿ”น Find where the logic goes wrong

This is much more powerful than adding "print()" everywhere.


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โšก Variables & Data Types in Java โญ

After understanding Java basics, the next important concept is Variables and Data Types. Every Java program stores and manipulates data, and this is done using variables. Letโ€™s understand everything step by step.

โœ… 1๏ธโƒฃ What is a Variable?

A variable is a container that stores data. Think of it like a box that holds values.

Example: int age = 25;

Here:
- int โ†’ data type
- age โ†’ variable name
- 25 โ†’ value stored in variable

Simple Structure:
data_type variable_name = value;

Example:
int number = 10;
double salary = 50000.50;
char grade = 'A';


โœ… 2๏ธโƒฃ Rules for Naming Variables

Java has some rules for variable names.

โœ” Must start with letter, _ or $
โœ” Cannot start with a number
โœ” Cannot use Java keywords

Valid examples:
- int age;
- double salary;
- String studentName;

Invalid examples:
- int 1age;
- double student-name;

โœ… 3๏ธโƒฃ Data Types in Java

Java has two main types of data types.

1๏ธโƒฃ Primitive Data Types
2๏ธโƒฃ Non-Primitive Data Types

๐Ÿ”น 4๏ธโƒฃ Primitive Data Types

Primitive types store simple values directly in memory. Java has 8 primitive data types.

- byte: 1 byte (e.g., byte a = 10;)
- short: 2 bytes (e.g., short b = 100;)
- int: 4 bytes (e.g., int age = 25;)
- long: 8 bytes (e.g., long population = 8000000000L;)
- float: 4 bytes (e.g., float price = 12.5f;)
- double: 8 bytes (e.g., double salary = 50000.75;)
- char: 2 bytes (e.g., char grade = 'A';)
- boolean: 1 bit (e.g., boolean isTrue = true;)

๐Ÿ”น 5๏ธโƒฃ Non-Primitive Data Types

Non-primitive types store references to objects.
Examples: String, Arrays, Classes, Objects, Interfaces

Example:
String name = "Java";
int[] numbers = {1, 2, 3, 4};


Difference:
- Primitive: Stores value, fixed size, faster.
- Non-Primitive: Stores reference, dynamic size, slightly slower.

๐Ÿ”น 6๏ธโƒฃ Type Casting

Type casting means converting one data type to another. There are two types.

โญ 1. Implicit Casting (Automatic): Smaller type โ†’ Larger type.
Example:
int number = 10;
double value = number;


โญ 2. Explicit Casting (Manual): Larger type โ†’ Smaller type.
Example:
double price = 99.99;
int value = (int) price; // Output: 99


๐Ÿ”น 7๏ธโƒฃ Constants in Java (final keyword)

A constant is a variable whose value cannot change. Java uses the final keyword.

Example:
final double PI = 3.14159;

Constants are usually written in UPPERCASE.

๐Ÿ”ฅ Example Program (Variables in Java)
class VariablesDemo {
public static void main(String[] args) {
int age = 25;
double salary = 50000.75;
char grade = 'A';
boolean isWorking = true;

System.out.println("Age: " + age);
System.out.println("Salary: " + salary);
System.out.println("Grade: " + grade);
System.out.println("Working: " + isWorking);
}
}


Output:
Age: 25
Salary: 50000.75
Grade: A
Working: true


โญ Common Interview Questions

1๏ธโƒฃ What are the 8 primitive data types in Java?
2๏ธโƒฃ What is the difference between primitive and non-primitive data types?
3๏ธโƒฃ What is type casting in Java?
4๏ธโƒฃ What is the difference between implicit and explicit casting?


๐Ÿ”ฅ Quick Revision

- Variables โ†’ containers for storing data.
- Primitive types: byte, short, int, long, float, double, char, boolean.
- Non-primitive types: String, Arrays, Objects, Classes.
- Type casting: Implicit โ†’ automatic; Explicit โ†’ manual.
- Constant: Created using the final keyword.


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๐Ÿญ. ๐—š๐—ฒ๐—ป๐—ฒ๐—ฟ๐—ฎ๐—น ๐——๐—ถ๐˜€๐—ฐ๐˜‚๐˜€๐˜€๐—ถ๐—ผ๐—ป ๐—š๐—ฟ๐—ผ๐˜‚๐—ฝ:

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