Updating and Deleting Data

You can now create and read data.

Next, you will learn how to:

• Update existing data
• Delete data

This completes the core operations of a database.

What You Will Do

You will:

• Update a user’s name
• Delete a user
• Verify the changes

Step 1: Update a User

Update database.py:

import sqlite3

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

user_id = 1
new_name = "John Updated"

cursor.execute(
    "UPDATE users SET name = ? WHERE id = ?",
    (new_name, user_id)
)

conn.commit()
conn.close()

print("User updated successfully")

Run It

python database.py

Step 2: Verify the Update

Use this code:

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()

Expected Output

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

Step 3: Delete a User

Now delete a user.

import sqlite3

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

user_id = 2

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

conn.commit()
conn.close()

print("User deleted successfully")

Run It

python database.py

Step 4: Verify Deletion

Run the query again:

cursor.execute("SELECT * FROM users")

Expected:

(1, 'John Updated')
(3, 'Bob')

Important Concepts

UPDATE

UPDATE users SET name = ? WHERE id = ?

• Changes existing data

DELETE

DELETE FROM users WHERE id = ?

• Removes data permanently

⚠️ Important Warning

Always use WHERE when updating or deleting.

Without it:

• You may update ALL rows
• You may delete ALL data

Why This Matters

You can now:

• Modify existing data
• Remove unwanted data
• Fully manage your database

This completes CRUD:

• Create
• Read
• Update
• Delete

Common Mistakes

• ❌ Forgetting WHERE
• ❌ Forgetting conn.commit()
• ❌ Using wrong ID

Summary

• You updated a user
• You deleted a user
• You completed full database operations

In the next lesson, you’ll use SQLAlchemy to make this easier inside Flask.