Relational Databases

Relational databases models data in the form of tables. Notably the tables are expected to have relationships with each other. Primary keys indicate unique entries in each table and foreign keys indicate relationships to unique entries in other tables it's related to.

The way most relational-databases are interacted with or queried is through SQL (Structured Query Language). SQL is a domain specific language used to interact with relational-databases.

Relational Database Management Systems

Concepts

Tables

A table is a collection of data organized into rows and columns. Each row is a record and each column is a field.

Cursors

A cursor allows processing the results of a query row by row within a database. In other words, a cursor is used to write the command returned from a query in a database. By using a cursor, you can iterate, or step through the results of a query and perform certain operations on each row.

In the simplest case a cursor can be defined in Python from your driver using the following syntax:

cursor.execute(query)

Where query is the command you wish the cursor to perform. The query must be written in SQL for the code to understand what you want the cursor to do.

query = ("INSERT INTO fruit (name, average_weight)"
  " VALUES (%s, %s))
data = ("Apple", 200)
cursor.execute(query, data)

In the above example, a query is defined to insert some entries into a database called fruit. This query defines the data that row will contain and uses a cursor to populate the data.

Naturally, the cursor class in Python has many attributes that you can use to manage data. More info about cursors in the Python Official Documentation

Interacting with Relational Databases in Python

Database Driver

When working with relational databases in python, in this case mysql a connector is needed to be installed called mysql-connector-python which is a driver to interact with the database in python. When writing python to interact with a database, that module also needs to be imported at the top of the file like below:

import mysql.connector

Connecting to Databases

cnx = mysql.connector.connect(user='root',
                                password='MyNewPass',
                                host='127.0.0.1',
                                database='',
                                auth_plugin='mysql_native_password')

Create Cursor

cursor = cnx.cursor()

Read Databases

query = ('SHOW DATABASES')
cursor.execute(query)
for row in cursor.fetchall():
    print(row)

Use DB

query = ("USE PLUTO")
cursor.execute(query)

Insert

import uuid
id = str(uuid.uuid4())
time = datetime.now().strftime('%Y-%m-%d %H:%M%S')
query = (f'INSERT INTO `posts` VALUES("{id}", "{time}")')
cursor.execute(query)

Delete Previous DB

Useful when recreating databases especially for testing functionality when state needs to be reset.

query = ("DROP DATABASE IF EXISTS `pluto`;")
cursor.execute(query)

Cleanup

cursor.close()
cnx.close()

Example: Library Management System

In module 12 of the PCDE course, there's a discussion about how to design a library management system as a prompt for coming up with their own database design. In this section, is a good example of different designs.

Database Containerization

See Database Containerization for more details. To summarize, containers are basic execution environments that are sandboxed and contained within sandboxed portions of an operating system. They are very useful for developing, testing and even deploying databases. Because of these properties, being able to consistently recreate the state of a database is very useful.

References

Web References

Note References