odiegoson / phase-3-03-sql-database-basics

Database Basics

Learning Goals

• Describe how relational databases store data in tables composed of columns and rows
• Understand database naming conventions
• Use the CREATE TABLE, ALTER TABLE and DROP TABLE commands
• Use the .help, .tables, and .schema commands

Introduction

We'll cover how to create and delete database tables in SQLite as well as how to add columns to an existing table.

Database Structure

Relational Databases like SQLite store data in a structure we refer to as a table. You can think of a table in a database a lot like you would a spreadsheet. We define specific columns in our table, and then we store any number of what we refer to as 'records' as rows in our database. A record is just information referring to one specific entity. For instance, if you had a table called "People" you could imagine a structure like this:

name	age	email
Bob	29	[e-mail]
Avi	28	[e-mail]
Adam	28	[e-mail]

Each column has a name, and each row contains the corresponding information about a person.

Thinking to what you've learned already about object oriented Ruby, you can also imagine how we might connect the idea of a table in SQL to a class in Ruby, and a record within a table to an instance of a class. For the example above, our Ruby representation of those same objects might look like this:

class Person
  attr_accessor :name, :age, :email

  def initialize(name, age, email)
    @name = name
    @age = age
    @email = email
  end
end

bob = Person.new("Bob", 29, "[e-mail]")
avi = Person.new("Avi", 28, "[e-mail]")
adam = Person.new("Adam", 28, "[e-mail]")

Note on Column Names

When we name columns in our database, there are a couple of conventions we will follow. The first is that we will always use lowercase letters when referring to column names in our database. SQLite isn't case sensitive about its commands or column names, but it is generally best practice for us to stick to lowercase for our column names.

The second convention we want to follow is more important. That is, when we have multiple words in a column name, we link them together using underscores rather than spaces. We call this convention snake case. So, for instance, if we wanted to be more specific with our email column above, we can name it email_address. If we wanted to split up name to first and last we might have columns called first_name and last_name.

Database Tables

In the following sections, we'll cover how to create, alter, and delete database tables. This reading is accompanied by a code along exercise that you can do in your terminal. You don't need to fork this repository, and there are no tests to pass. Follow along with the reading and code along instructions.

Create Table

When we create a new database, it comes like a sort of blank slate. We can then create a table inside our database using the following statement:

CREATE TABLE table_name;

But before we're able to store any actual data in a table, we'll need to define the columns in the table as well as the specific type of data each column will store.

Let's give it a shot. For the purposes of this code along, you'll be typing these commands into your terminal.

Code Along 1: Creating a Table

In the terminal let's create our new database and start sqlite3 by running the following:

$ sqlite3 pet_database.db

Now, at our SQLite prompt, let's create our table:

CREATE TABLE cats;

You should see the following error:

Error: near ";": syntax error

SQLite expects us to include at least some definition of the structure of this table as well. In other words, when we create database tables, we need to specify some column names, along with the type of data we are planning to store in each column. More on data types later.

Let's try that table statement again:

CREATE TABLE cats (
  id INTEGER PRIMARY KEY,
  name TEXT,
  age INTEGER
);

Let's break down the above code:

1. Use the CREATE TABLE command to create a new table called "cats".
2. Include a list of column names along with the type of data they will be storing. TEXT means we'll be storing plain old text, INTEGER means we'll store a number. Note that the use of capitalization is arbitrary, but it is a convention to help separate the SQL commands from the names we make up for our tables and columns.
3. Every table we create, regardless of the other column names and data types, should be defined with an id INTEGER PRIMARY KEY column, including the integer data type and primary key designation. Our SQLite database tables must be indexed by a number. We want each row in our table to have a number, which we'll call "id", just like in an Excel spreadsheet. Numbering our table rows makes our data that much easier to access, update, and organize. SQLite comes with a data type designation called "Primary Key". Primary keys are unique and auto-incrementing, meaning they start at 1 and each new row automatically gets assigned the next numeric value.

Okay, let's check and make sure that we successfully created that table. To do this we'll be using SQL commands. To get a complete list of commands, you can type .help into the sqlite prompt.

Wow, that's a lot. Don't worry too much about all of these different commands right now. Just know that you can always use .help to check out the available options.

Okay, let's check out our new table. To list all the tables in the database we'll use the .tables command. Type it into the sqlite prompt and hit enter, and you should see our cats table listed.

We can look at the structure, or "schema", of our database (i.e. the tables and their columns + column data types) with the .schema command:

sqlite> .schema
CREATE TABLE cats (
  id INTEGER PRIMARY KEY,
  name TEXT,
  age INTEGER
);

You can also use the SQLite VSCode extension, or DB Browser for SQLite, to see a visual representation of the table. There won't be much to look at yet, since we haven't added any data to the table; but you will be able to see the structure of the table.

Let's move on to altering our table.

Alter Table

Let's say that, after creating a database and creating a table to live inside that database, we decide we want to add or remove a column. We can do so with the ALTER TABLE statement.

Code Along 2: Adding, Removing and Renaming Columns

Let's say we want to add a new column, breed, to our cats table:

ALTER TABLE cats ADD COLUMN breed TEXT;

Let's check out our schema now:

sqlite> .schema
CREATE TABLE cats (
  id INTEGER PRIMARY KEY,
  name TEXT,
  age INTEGER,
  breed TEXT
);

Notice that the ALTER statement isn't here, but instead SQLite has updated our original CREATE statement. The schema reflects the current structure of the database, which is reflected as the CREATE statement necessary to create that structure.

Unfortunately, altering a column name and/or deleting a column can be tricky in SQLite3. There are workarounds, however. We're not going to get into that right now, but you can explore the documentation on this topic.

Fortunately, SQLite still supports most of what we'll need one way or another. For now, if you need to change a column name, it's best to simply delete the table and re-create it.

Drop Table

Lastly, we'll discuss how to delete a table from a database with the DROP TABLE statement.

Code Along 3: Deleting a Table

Deleting a table is very simple:

DROP TABLE cats;

And that's it! You can exit out of the sqlite prompt with the .quit command.

Resources

• SQL keywords