SQL (Structured Query Language) is a language for managing data in a database. Unlike some other programming languages, it's only used for one thing: talking to databases. Thus, you might hear it referred to as a "special purpose", or "domain specific" programming language. This means that you won't be using SQL to write the next big web app, but you might use it to interact with the database that powers it.

Even though SQL has just one purpose, it is used by many different database systems such as MySQL, PostgreSQL, or the system we'll be using in this course: SQLite. There are a handful of things other SQL systems like Postgres, or MySQL can do that are not supported by SQLite. Every database system has its own strengths and weaknesses, and as you learn more about them you should evaluate them thoughtfully when deciding which to use for what purpose. For us, SQLite provides a low barrier to entry, and is simple to get up and running.

Why do we need to master SQL? As a full-stack web developer, you'll frequently be working with databases to manage the data associated with your applications. Think about the web apps you're familiar with. Facebook saves user data and stores––or persists––your associations to your friends. Amazon has an enormous database of items for sale. The New York Times has a storage system for all of their articles. Most of the sites you interact with every day, and consequently most of the sites you'll work on or build, need databases to persist data.

We've already seen how we can build Ruby programs that model real-world objects and environments. With database management skills, we'll learn to store representations of the Ruby objects our programs create and retrieve them at the appropriate time. We’ll also learn how to connect our Ruby (and even later, Ruby on Rails) applications to our databases.

For example, a basic web application might have many users. So far, we've learned how to build a Ruby User class that produces user objects. But, we don't yet know how to store those users and their details. If a user signs up for our app and we proceed to lose all their information immediately, how will we know if an existing user is signing back into our app? We need a way to take our Ruby objects, store them in a database and retrieve them at the appropriate time.

This topic will cover how to use and navigate databases based on SQL. By the end of this topic, you’ll be able to:

• Create SQLite3 databases.
• Create, update, select, and delete data from database tables.
• Relate data within a given database.
• Write SQL code in both your command line and your text editor and execute the code against a database.
• Write Ruby programs that talk to and save data to your databases.