Overview

5.1. Overview#

5.1.1. What is a Database?#

A database stores data relating to a particular subject. For example, you can have a database storing all the information on staff in a company.

Databases store data as a collection of relations (tables). There are usually relationships between these relations in the database.

Below is an example of a shopping database.

../../_images/shopping-database.png

In this database, we have 3 tables.

  • The customers table tells us about the customers that come to the shop

  • The products table tells us about the items that the shop is selling

  • The transactions table tells us which customers have bought which items, and how much they have purchased.

5.1.2. Querying a Database#

To extract information from a database we construct a query.

A query is an instruction that tells the database what to do. For example, we can get a database to display a table, or we can tell the database to add new data.

To give our instructions to the database in a way it can understand, we need to formulate our query with a particular structure. For this, we will be using SQL (Structured Query Language). Like Python, SQL has its own rules for how you should write commands.

Here is an example of a query that displays the customers table.

SELECT *
FROM customers;
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