Raw queries when needed in Express - Mini Project: Build & Apply

Raw Queries When Needed in Express

📖 Scenario: You are building a simple Express app that needs to fetch user data from a database. Sometimes, you want to use raw SQL queries directly for more control.

🎯 Goal: Create an Express app that connects to a database and uses a raw SQL query to get users with age over a certain limit.

📋 What You'll Learn

Create a database connection variable called db

Create a variable ageLimit set to 30

Write a raw SQL query using db.query to select users older than ageLimit

Send the query result as JSON in the Express route handler

💡 Why This Matters

🌍 Real World

Raw SQL queries are useful when you need precise control over database operations beyond what an ORM offers. This is common in real-world apps that need optimized or complex queries.

💼 Career

Many backend developer jobs require knowledge of how to safely run raw queries in Express apps to handle special cases or performance tuning.

1

Set up database connection

Create a variable called db that requires the mysql2/promise module and connects to a database with host 'localhost', user 'root', password 'password', and database 'testdb'.

Express

# Your code here

Hint

Use mysql2/promise to create a connection pool with the given credentials.

2

Add age limit variable

Create a variable called ageLimit and set it to 30.

Express

const mysql = require('mysql2/promise');
const db = mysql.createPool({
  host: 'localhost',
  user: 'root',
  password: 'password',
  database: 'testdb'
});

// Your code here

Hint

Just create a constant named ageLimit and assign it the number 30.

3

Write raw SQL query in Express route

Create an Express app with express(). Add a GET route at '/users' that uses await db.query with the raw SQL 'SELECT * FROM users WHERE age > ?' and passes ageLimit as a parameter. Store the result in a variable called rows.

Express

const mysql = require('mysql2/promise');
const db = mysql.createPool({
  host: 'localhost',
  user: 'root',
  password: 'password',
  database: 'testdb'
});

const ageLimit = 30;

// Your code here

Hint

Use db.query with a parameterized query to safely insert ageLimit.

4

Start the Express server

Add app.listen on port 3000 with a callback that logs 'Server running on port 3000'.

Express

const mysql = require('mysql2/promise');
const db = mysql.createPool({
  host: 'localhost',
  user: 'root',
  password: 'password',
  database: 'testdb'
});

const ageLimit = 30;

const express = require('express');
const app = express();

app.get('/users', async (req, res) => {
  const [rows] = await db.query('SELECT * FROM users WHERE age > ?', [ageLimit]);
  res.json(rows);
});

// Your code here

Hint

Use app.listen to start the server on port 3000 and log a message.

Practice

1. What is the main reason to use raw queries in Express applications?

easy

A. To write direct SQL commands when query builders are insufficient

B. To automatically generate SQL queries without any code

C. To avoid using any SQL and rely only on JavaScript

D. To speed up the server by skipping database calls

Solution

1. Step 1: Understand raw queries purpose

   Raw queries allow writing direct SQL commands for complex or special cases.

2. Step 2: Compare with other options

   Options A, B, and C are incorrect because raw queries do not skip database calls, auto-generate SQL, or avoid SQL.

3. Final Answer:

   To write direct SQL commands when query builders are insufficient -> Option A

4. Quick Check:

   Raw queries = direct SQL use [OK]

Hint: Raw queries = direct SQL for special cases [OK]

Common Mistakes:

• Thinking raw queries auto-generate SQL
• Believing raw queries avoid SQL entirely
• Assuming raw queries skip database calls

2. Which of the following is the correct way to use a raw query with parameters in Express using a SQL library?

easy

A. db.query('SELECT * FROM users WHERE id = ' + userId)

B. db.query('SELECT * FROM users WHERE id = ?', [userId])

C. db.query('SELECT * FROM users WHERE id = $userId')

D. db.query('SELECT * FROM users WHERE id = :userId')

Solution

1. Step 1: Identify parameterized query syntax

   Using '?' with an array of parameters is the standard safe way to pass values.

2. Step 2: Check other options

   db.query('SELECT * FROM users WHERE id = ' + userId) concatenates strings (unsafe). Options C and D use placeholders not standard in many Express SQL libraries.

3. Final Answer:

   db.query('SELECT * FROM users WHERE id = ?', [userId]) -> Option B

4. Quick Check:

   Use '?' + params array for safe raw queries [OK]

Hint: Use '?' with array for parameters in raw queries [OK]

Common Mistakes:

• Concatenating variables directly into query strings
• Using unsupported placeholder syntax
• Not passing parameters as an array

3. Given the code snippet:

const result = await db.query('SELECT * FROM products WHERE price > ?', [100]);
console.log(result.rows.length);

What will be logged if there are 5 products priced over 100?

medium

A. 5

B. undefined

C. 0

D. An error will occur

Solution

1. Step 1: Understand the query and parameters

   The raw query selects products with price greater than 100, passing 100 safely as a parameter.

2. Step 2: Interpret the result object

   Assuming the database library returns an object with a 'rows' array, its length will be the number of matching products, which is 5.

3. Final Answer:

   5 -> Option A

4. Quick Check:

   Rows length = number of matching products [OK]

Hint: Result rows length = count of matched records [OK]

Common Mistakes:

• Expecting result directly as array without .rows
• Confusing parameter placement
• Assuming query returns undefined or error

4. Identify the error in this raw query usage:

const userId = 10;
const result = await db.query('SELECT * FROM users WHERE id = ' + userId);

medium

A. The parameter should be passed as an object, not concatenated

B. The query syntax is invalid because of missing quotes

C. It risks SQL injection by concatenating variables directly

D. The query will always return zero results

Solution

1. Step 1: Analyze query construction

   The query concatenates userId directly into the string, which is unsafe and can cause SQL injection.

2. Step 2: Understand best practice

   Parameters should be passed separately using placeholders to avoid injection risks.

3. Final Answer:

   It risks SQL injection by concatenating variables directly -> Option C

4. Quick Check:

   Never concatenate variables directly in raw queries [OK]

Hint: Avoid string concatenation; use placeholders [OK]

Common Mistakes:

• Ignoring SQL injection risks
• Thinking concatenation is safe for numbers
• Not using parameterized queries

5. You need to update multiple user records with different emails using raw queries in Express. Which approach is best to safely perform this?

hard

A. Use raw queries without parameters for faster execution

B. Concatenate all updates into one big query string and run once

C. Update users one by one by concatenating values directly

D. Use a loop with parameterized raw queries for each user update

Solution

1. Step 1: Consider safety and flexibility

   Using a loop with parameterized queries ensures each update is safe and avoids SQL injection.

2. Step 2: Evaluate other options

   Concatenating all updates risks injection and syntax errors; skipping parameters is unsafe; direct concatenation is insecure.

3. Final Answer:

   Use a loop with parameterized raw queries for each user update -> Option D

4. Quick Check:

   Loop + parameters = safe batch updates [OK]

Hint: Loop with parameters for multiple safe updates [OK]

Common Mistakes:

• Trying to build one big concatenated query
• Ignoring parameterization for speed
• Concatenating user data directly