Python & SQL – Interacting with Relational Databases - Learn PYTHON-BACKEND | Revochamp

Name: Python & SQL – Interacting with Relational Databases
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Python and the SQL Standard

Python interacts with relational databases through a standard specification called DB-API 2.0 (PEP 249). This ensures that whether you are using SQLite, MySQL, or PostgreSQL, the basic methods like connect(), execute(), and fetchone() remain consistent across different database drivers.

Working with SQLite (Built-in)

Python comes with the sqlite3 module out of the box. It is perfect for local storage, mobile app testing, or small-scale automation tools. Since it's a file-based database, no server installation is required.

PythonRead-only

import sqlite3

# 1. Connect to a database file (creates it if it doesn't exist)
conn = sqlite3.connect('local_storage.db')
cursor = conn.cursor()

# 2. Create a table
cursor.execute('''CREATE TABLE IF NOT EXISTS projects 
               (id INTEGER PRIMARY KEY, name TEXT, stars INTEGER)''')

# 3. Insert data safely
project_data = ('Revochamp', 500)
cursor.execute("INSERT INTO projects (name, stars) VALUES (?, ?)", project_data)

# 4. Commit and close
conn.commit()
conn.close()

import sqlite3

# 1. Connect to a database file (creates it if it doesn't exist)
conn = sqlite3.connect('local_storage.db')
cursor = conn.cursor()

# 2. Create a table
cursor.execute('''CREATE TABLE IF NOT EXISTS projects 
               (id INTEGER PRIMARY KEY, name TEXT, stars INTEGER)''')

# 3. Insert data safely
project_data = ('Revochamp', 500)
cursor.execute("INSERT INTO projects (name, stars) VALUES (?, ?)", project_data)

# 4. Commit and close
conn.commit()
conn.close()

The Database Connection Workflow

Every SQL interaction in Python follows a specific hierarchy of objects: the Connection manages the link to the database, while the Cursor acts as a pointer to execute commands and fetch results.

Parameterized Queries vs. Injection

As a Technical Lead, security is a priority. SQL Injection occurs when user input is directly concatenated into a query string. Python drivers prevent this by using placeholders (? for SQLite, %s for others).

PythonRead-only

user_id = "1 OR 1=1" # A common injection attack

# ❌ DANGEROUS
# cursor.execute(f"SELECT * FROM users WHERE id = {user_id}")

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

user_id = "1 OR 1=1" # A common injection attack

# ❌ DANGEROUS
# cursor.execute(f"SELECT * FROM users WHERE id = {user_id}")

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

Common DB-API Methods

Method	Purpose	Returns
execute()	Runs a single SQL command	Cursor object
executemany()	Runs a command against a list of data	None
fetchone()	Retrieves the next row of a query result	Tuple or None
fetchall()	Retrieves all remaining rows	List of Tuples
commit()	Saves all current changes to the DB	None
rollback()	Undoes changes since the last commit	None

Try it yourself

import sqlite3

# Create an in-memory database for testing
conn = sqlite3.connect(':memory:')
cur = conn.cursor()

cur.execute("CREATE TABLE dev(name, lang)")
cur.execute("INSERT INTO dev VALUES (?, ?)", ("Kishore", "Python"))

# Query data
cur.execute("SELECT * FROM dev")
row = cur.fetchone()

print(f"Developer: {row[0]}, Language: {row[1]}")

Test Your Knowledge

of 3

Which object is responsible for executing SQL commands and fetching results?

Connection

Cursor

Driver

ORM

of 3

Which placeholder does SQLite use for parameterized queries?

:val

of 3

What command must be called to ensure data changes are saved to the database?

save()

push()

commit()

close()

Frequently Asked Questions

When should I use SQLite vs. a server-based SQL like PostgreSQL?

Use SQLite for local apps, prototypes, or read-heavy apps with low concurrency. Use PostgreSQL or MySQL for web applications with many simultaneous users and high write volumes.

What is 'conn.commit()' and why do I need it?

SQL databases use transactions. When you Insert or Update data, the changes are 'pending' in a temporary state. Calling commit() makes those changes permanent in the database file.

Can I return results as a Dictionary instead of a Tuple?

Yes. In SQLite, you can set 'conn.row_factory = sqlite3.Row'. This allows you to access columns by name (e.g., row['name']) instead of index (e.g., row[0]).

import sqlite3 # 1. Connect to a database file (creates it if it doesn't exist) conn = sqlite3.connect('local_storage.db') cursor = conn.cursor() # 2. Create a table cursor.execute('''CREATE TABLE IF NOT EXISTS projects (id INTEGER PRIMARY KEY, name TEXT, stars INTEGER)''') # 3. Insert data safely project_data = ('Revochamp', 500) cursor.execute("INSERT INTO projects (name, stars) VALUES (?, ?)", project_data) # 4. Commit and close conn.commit() conn.close()

user_id = "1 OR 1=1" # A common injection attack # ❌ DANGEROUS # cursor.execute(f"SELECT * FROM users WHERE id = {user_id}") # ✅ SAFE cursor.execute("SELECT * FROM users WHERE id = ?", (user_id,))

Method

Purpose

Returns

execute()

Runs a single SQL command

Cursor object

executemany()

Runs a command against a list of data

None

fetchone()

Retrieves the next row of a query result

Tuple or None

fetchall()

Retrieves all remaining rows

List of Tuples

commit()

Saves all current changes to the DB

None

rollback()

Undoes changes since the last commit

None

Try it yourself

import sqlite3

# Create an in-memory database for testing
conn = sqlite3.connect(':memory:')
cur = conn.cursor()

cur.execute("CREATE TABLE dev(name, lang)")
cur.execute("INSERT INTO dev VALUES (?, ?)", ("Kishore", "Python"))

# Query data
cur.execute("SELECT * FROM dev")
row = cur.fetchone()

print(f"Developer: {row[0]}, Language: {row[1]}")

Test Your Knowledge

of 3

Which object is responsible for executing SQL commands and fetching results?

Connection

Cursor

Driver

ORM

of 3

Which placeholder does SQLite use for parameterized queries?

:val

of 3

What command must be called to ensure data changes are saved to the database?

save()

push()

commit()

close()

Frequently Asked Questions

When should I use SQLite vs. a server-based SQL like PostgreSQL?

Use SQLite for local apps, prototypes, or read-heavy apps with low concurrency. Use PostgreSQL or MySQL for web applications with many simultaneous users and high write volumes.

What is 'conn.commit()' and why do I need it?

SQL databases use transactions. When you Insert or Update data, the changes are 'pending' in a temporary state. Calling commit() makes those changes permanent in the database file.

Can I return results as a Dictionary instead of a Tuple?

Yes. In SQLite, you can set 'conn.row_factory = sqlite3.Row'. This allows you to access columns by name (e.g., row['name']) instead of index (e.g., row[0]).

Python & SQL – Interacting with Relational Databases

Python & SQL – Interacting with Relational Databases

Python and the SQL Standard

Working with SQLite (Built-in)

The Database Connection Workflow

Parameterized Queries vs. Injection

Common DB-API Methods

Try it yourself

Test Your Knowledge

Frequently Asked Questions

Need help?

Python & SQL – Interacting with Relational Databases

Python & SQL – Interacting with Relational Databases

Python and the SQL Standard

Working with SQLite (Built-in)

The Database Connection Workflow

Parameterized Queries vs. Injection

Common DB-API Methods

Try it yourself

Test Your Knowledge

Frequently Asked Questions

Need help?