Querying Data

You have stored data in your database.

Now you need to retrieve it.

This is called querying.

What You Will Do

You will:

• Read all users
• Read a single user by ID
• Display results

Step 1: Get All Users

Update database.py:

import sqlite3

conn = sqlite3.connect("database.db")
cursor = conn.cursor()

cursor.execute("SELECT * FROM users")

users = cursor.fetchall()

for user in users:
    print(user)

conn.close()

Run It

python database.py

Output

(1, 'John')
(2, 'Alice')
(3, 'Bob')

Step 2: Get One User by ID

Now let’s fetch a specific user.

Update your code:

import sqlite3

conn = sqlite3.connect("database.db")
cursor = conn.cursor()

user_id = 1

cursor.execute("SELECT * FROM users WHERE id = ?", (user_id,))

user = cursor.fetchone()

print(user)

conn.close()

Run It

python database.py

Output

(1, 'John')

What Changed?

WHERE Clause

SELECT * FROM users WHERE id = ?

• WHERE filters the data
• Only returns matching rows

fetchone()

• Returns one row
• Instead of a list

Step 3: Try Different IDs

Change:

user_id = 2

Run again.

You should see:

(2, 'Alice')

Important Concept

You now know two types of queries:

• fetchall() → multiple rows
• fetchone() → single row

Why This Matters

This is how real apps work:

• View one user profile
• Load specific data
• Handle dynamic requests

Common Mistakes

• ❌ Forgetting (user_id,) (must be a tuple)
• ❌ Using fetchall() when expecting one item
• ❌ Wrong column name

Summary

• You retrieved all users
• You retrieved a single user
• You used WHERE to filter data

In the next lesson, you’ll update and delete data.