Oleksandr Starshynov w3 databases

Week3/assignmentWeek3/ex1 - normalization.md

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+# Exercise 1: SQL Normalization – Dinner Club
+
+## Step 1: Violation of 1NF (First Normal Form)
+
+**1NF Rule**: Each field must contain only **atomic** (indivisible) values.
+
+### ❌ Violations of 1NF:
+- **`food_code`** and **`food_description`** contain **multiple values** separated by commas (e.g., `"C1, C2"`, `"Curry, Cake"`).
+- **`dinner_date`** has **inconsistent formats** (e.g., `"2020-03-15"`, `"20-03-2020"`, `"Mar 25 '20"`), violating domain consistency.
+
+---
+
+## Step 2: Recognized Entities
+
+Based on the table, we can identify the following entities:
+
+1. **Member** – identified by `member_id`.
+2. **Dinner** – identified by `dinner_id`.
+3. **Venue** – identified by `venue_code`.
+4. **Food** – identified by `food_code`.
+5. **Dinner Participation** – mapping between members and dinners (many-to-many).
+6. **Dinner Food** – mapping between dinners and foods served (many-to-many).
+
+---
+
+## Step 3: 3NF-Compliant Tables and Attributes
+
+### 1. **Members**
+| Column Name     | Type        |
+|-----------------|-------------|
+| member_id (PK)  | INTEGER     |
+| member_name     | TEXT        |
+| member_address  | TEXT        |
+
+---
+
+### 2. **Venues**
+| Column Name         | Type    |
+|---------------------|---------|
+| venue_code (PK)     | TEXT    |
+| venue_description   | TEXT    |
+
+---
+
+### 3. **Dinners**
+| Column Name     | Type      |
+|-----------------|-----------|
+| dinner_id (PK)  | TEXT      |
+| dinner_date     | DATE      |
+| venue_code (FK) | TEXT      |
+
+---
+
+### 4. **Foods**
+| Column Name        | Type    |
+|--------------------|---------|
+| food_code (PK)     | TEXT    |
+| food_description   | TEXT    |
+
+---
+
+### 5. **Dinner_Participation** (Many-to-Many: members attend multiple dinners)
+| Column Name        | Type    |
+|--------------------|---------|
+| member_id (FK)     | INTEGER |
+| dinner_id (FK)     | TEXT    |
+
+**Primary Key**: (member_id, dinner_id)
+
+---
+
+### 6. **Dinner_Food** (Many-to-Many: dinners serve multiple foods)
+| Column Name        | Type    |
+|--------------------|---------|
+| dinner_id (FK)     | TEXT    |
+| food_code (FK)     | TEXT    |
+
+**Primary Key**: (dinner_id, food_code)
+
+---
+
+## Summary of Normalization
+
+- The **original table** violates **1NF** due to multi-valued fields and inconsistent formats.
+- We **decomposed** the table into **six 3NF-compliant tables** by:
+  - Ensuring atomicity of values,
+  - Removing transitive and partial dependencies,
+  - Isolating repeating groups into separate relations.

Week3/assignmentWeek3/ex2 - SQL Transactions/transaction.js

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+import mysql from "mysql2/promise";
+
+async function performTransfer() {
+  const connection = await mysql.createConnection({
+    host: "localhost",
+    user: "hyfuser",
+    password: "hyfpassword",
+    database: "w3_assignment",
+  });
+
+  const fromAccount = 101;
+  const toAccount = 102;
+  const amount = 1000;
+  const date = "2025-06-02";
+
+  try {
+    // Start transaction 
+    await connection.beginTransaction();
+    console.log("Transaction started");
+
+    const transactionFrom = {
+      account_number: fromAccount,
+      amount: -amount,
+      changed_date: date,
+      remark: `Transfer to account #${toAccount}`,
+    };
+
+    const transactionTo = {
+      account_number: toAccount,
+      amount: amount,
+      changed_date: date,
+      remark: `Transfer from account #${fromAccount}`,
+    };
+
+    const [availableBalanceResult] = await connection.query(
+      "SELECT balance FROM account WHERE account_number = ?",
+      [fromAccount]
+    );
+
+    const availableBalance = availableBalanceResult[0]?.balance;
+
+    if (availableBalance < amount) {
+      throw new Error("Insufficient balance to perform the transfer");
+    }
+
+    console.log(`Account #${fromAccount} available balance: ${availableBalance}`);
+
+    // Deduct from sender
+    await connection.query("INSERT INTO account_changes SET ?", transactionFrom);
+    await connection.query(
+      "UPDATE account SET balance = balance + ? WHERE account_number = ?",
+      [transactionFrom.amount, fromAccount]
+    );
+    console.log(`$${amount} deducted from account #${fromAccount}`);
+
+    // Add to receiver
+    await connection.query("INSERT INTO account_changes SET ?", transactionTo);
+    await connection.query(
+      "UPDATE account SET balance = balance + ? WHERE account_number = ?",
+      [transactionTo.amount, toAccount]
+    );
+    console.log(`$${amount} added to account #${toAccount}`);
+
+    await connection.commit();
+    console.log("Transaction completed successfully.");
+  } catch (error) {
+    await connection.rollback();
+    console.error("Transaction failed:", error.message);
+  } finally {
+    await connection.end();
+    console.log("Connection closed"); 
+  }
+}

Week3/assignmentWeek3/ex2 - SQL Transactions/transactions-create-tables.js

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+import mysql from 'mysql2/promise';
+
+async function createTables() {
+  const connection = await mysql.createConnection({
+    host: 'localhost',
+    user: 'hyfuser',
+    password: 'hyfpassword',
+    database: 'w3_assignment',
+  });
+
+  try {
+    await connection.beginTransaction();
+
+    await connection.query(`
+      CREATE TABLE IF NOT EXISTS account (
+        account_number INT PRIMARY KEY,
+        balance DECIMAL(12, 2) NOT NULL
+      )
+    `);
+
+    await connection.query(`
+      CREATE TABLE IF NOT EXISTS account_changes (
+        change_number INT AUTO_INCREMENT PRIMARY KEY,
+        account_number INT,
+        amount DECIMAL(12, 2) NOT NULL,
+        changed_date TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
+        remark TEXT,
+        FOREIGN KEY (account_number) REFERENCES account(account_number)
+      )
+    `);
+
+    await connection.commit();
+    console.log('Tables created successfully');
+  } catch (err) {
+    await connection.rollback();
+    console.error('Error creating tables:', err);
+  } finally {
+    await connection.end();
+  }
+}
+
+createTables();

Week3/assignmentWeek3/ex2 - SQL Transactions/transactions-insert-values.js

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+import mysql from 'mysql2/promise';
+
+async function insertValues() {
+  const connection = await mysql.createConnection({
+    host: 'localhost',
+    user: 'hyfuser',
+    password: 'hyfpassword',
+    database: 'w3_assignment',
+  });
+
+  try {
+    await connection.beginTransaction();
+
+    await connection.query(`
+      INSERT INTO account (account_number, balance) VALUES
+      (101, 5000.00),
+      (102, 3000.00)
+    `);
+
+    await connection.query(`
+      INSERT INTO account_changes (account_number, amount, remark) VALUES
+      (101, 5000.00, 'Initial deposit'),
+      (102, 3000.00, 'Initial deposit')
+    `);
+
+    await connection.commit();
+    console.log('Values inserted successfully');
+  } catch (err) {
+    await connection.rollback();
+    console.error('Error inserting values:', err);
+  } finally {
+    await connection.end();
+  }
+}
+
+insertValues();

Week3/assignmentWeek3/ex3 - SQL injection.md

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+1. - Give an example of a value that can be passed as name and code that would take advantage of SQL-injection and ( fetch all the records in the database)
+- answer:
+- SELECT Population FROM Countries WHERE Name = '' OR '1'='1' and code = '' OR '1'='1'
+- I think it would work sinse '1'='1' is always true the query will return all rows in the table, ignoring filtering by name and code.
+
+2. 
+```
+function getPopulation(Country, name, code, cb) {
+  const allowedTables = ['Countries', 'OtherValidTable']; 
+  if (!allowedTables.includes(Country)) {
+    return cb(new Error("Invalid table name"));
+  }
+
+  const sql = `SELECT Population FROM ${Country} WHERE Name = ? AND code = ?`;
+
+  conn.query(sql, [name, code], function (err, result) {
+    if (err) return cb(err);
+    if (result.length === 0) return cb(new Error("Not found"));
+    cb(null, result[0].Population);
+  });
+}
+```