Introduction To SQL and Databases

Objectives

Connect to a PostgreSQL database using psql
Use SQL to create a database and tables
Use SQL to insert, select, update, and delete data
Understand what Primary Keys are

How are databases used in the wild?

For learning and testing purposes, we will be using Postgres on the same machine that our web server is running. In the real world, your database will be on a separate machine, called a database server.

A database server is a computer or group of computers that is dedicated to storing your data and handling remote requests to retreive that data. Even in a very simple configuration, the database server will have at least 1 backup machine that keeps an exact copy of the database just in case the main database server goes down.

psql

Today we're using PostgreSQL, often called Postgres. Postgres is based off an older database system called Ingres. That's where the name comes from. We're using a "post-Ingres" database.

If you installed Postgres.app you have access to psql from the elephant icon at the top of the screen:

If you are using the command line:

In your terminal, type psql.

psql is a command line tool to interact with postgres databases, by default it connects to the localhost database with the name of the current user

psql has some of it's own command which begin with \

List all of the available databases:

\list

List all of the available tables in the current database:

\dt

Check you connection information:

\conninfo

There are lots of other commands which you can see with:

\?

Use \q to exit the help screen

Note that all psql commands start with \

At this point we should have a database with no tables in it. So now we need to create tables - using SQL (NOT to be confused with the psql app itself)

SQL: Structured Query Language

A Brief History of Databases

Before the notion of an RDBMS and a standard language for querying that data was created, there were many database venders. Each vendor had different ways of storing data and very different ways of retreiving the data afterwards. Moving data from one system to another was very costly. Luckly in the 1970s SQL was created and later turned into a standard. Modern relational databases are now based on the SQL standard, so moving from Postgres to Oracle is not nearly as much of a challenge as it used to be.

CRUD

Stands for Create, Read, Update and Destroy. This is the lifecycle of data in an applicatoin. In SQL, CRUD can be mapped to the following INSERT, SELECT, UPDATE, DELETE. We will walk through examples of in this section.

Creating a Database

Most database products have the notion of separate databases. Lets create one for the lesson.

CREATE DATABASE testdb;

ALL SQL COMMANDS MUST BE ENDED WITH A SEMICOLON IN THE PSQL SHELL

Connect to a database

\connect testdb

Once we connect, our command prompt should look similar to this: testdb=#

List our tables in a database:

\dt

CREATE-ing a Table

This is an example of a students table. (We will talk about the primary key soon.)

CREATE TABLE students (
    id SERIAL PRIMARY KEY,
    name TEXT,
    phone VARCHAR(15),
    email TEXT
);

Check that it's there:

\dt

Look at the table structure

 \d+ students

INSERT-ing Data

INSERT INTO students
(name, phone, email)
VALUES
('William Smith', '(415)555-5555', '[email protected]');

INSERT INTO students
(name, phone, email)
VALUES
('Bob Jones', '(415)555-5555', '[email protected]');

SELECT-ing Data

SELECT * FROM students;

SELECT * FROM students WHERE name = 'Bob Jones';

SELECT id, name FROM students;

UPDATE-ing Data

UPDATE students SET email='[email protected]' WHERE name = 'Bob Jones';

DELETE-ing Data

DELETE from students WHERE name = 'Mary';
->DELETE 0

DELETE from students WHERE email = '[email protected]';
->DELETE 1

DROP-ing a Table

DROP TABLE students;

Database Schema Design

For this lesson, how to design a complete database system is out of scope. We will discuss a few things here though.

The schema of the database is the set of create table commands that specify what the tables are and how they relate to each other. For our very simple database example, here is the schema:

testdb=# \d+ students
                                                    Table "public.students"
 Column |         Type          |                       Modifiers
--------+-----------------------+-------------------------------------------------------
 id     | integer               | not null default nextval('students_id_seq'::regclass)
 name   | text                  |
 phone  | character varying(15) |
 email  | text                  |

What is a Primary Key?

It denotes an attribute on a table that can uniquely identify the row.

What does SERIAL Do?

SERIAL tells the database to automatically assign the next unused integer value to id whenever we insert into the database and do not specify id. In general, if you have a column that is set to SERIAL, it is a good idea to let the database assign the value for you.

What is a foreign key?

It provides a reference to a primary key in another table.

CREATE TABLE courses (
    id SERIAL PRIMARY KEY,
    course TEXT
);

CREATE TABLE students (
    id SERIAL PRIMARY KEY,
    name TEXT,
    phone VARCHAR(15),
    email TEXT,
    course_id INTEGER REFERENCES courses
);

Data Types

Similar to how Ruby has types of data, SQL defines types that can be stored in the DB. Here are some common ones:

Serial
Integer
Numeric // Numbers are exact, no rounding error
Float // Rounding error is possible, but operations are faster than Numeric
Text, Varchar
Timestamp
Boolean (True or False)

Exercise Time

Design a table for a movie database. Discuss a few things that a movie table may have. Choose the appropriate data type for the data. Make the CREATE TABLE command and execute it in psql. Use \dt to verify that the table was created. Once you're satisfied that the table is there, get rid of it using the DROP TABLE command. Use \dt again to make sure that the table has been dropped.

Selecting

A select statement allows you to get data from the database. Here are the docs on select. Also, postgres a good tutorial on select. I'd recommend looking at the tutorial sometime after the lesson.

Create a new database to hold a movies table:

CREATE DATABASE testdb;

Connect to the new database:

\connect testdb;

Given this table:

CREATE TABLE movies (
  id SERIAL PRIMARY KEY,
  title TEXT,
  description TEXT,
  rating INTEGER
);

And these insert statements:

INSERT INTO movies (title, description, rating) VALUES('Cars', 'a movie', 9);
INSERT INTO movies (title, description, rating) VALUES('Back to the Future', 'another movie', 9);
INSERT INTO movies (title, description, rating) VALUES('Dude Wheres My Car', 'probably a bad movie', 3);
INSERT INTO movies (title, description, rating) VALUES('Godfather', 'good movie', 10);
INSERT INTO movies (title, description, rating) VALUES('Mystic River', 'did not see it', 7);
INSERT INTO movies (title, description, rating) VALUES('Argo', 'a movie', 8);
INSERT INTO movies (title, description, rating) VALUES('Gigli', 'really bad movie', 1);

This will select all the attributes from the movies table unconditionally. Make sure not to forget the ; at the end of each statement. In SQL, semi colons are required to terminate statements.

SELECT * FROM movies;
`

We may not want all attribues though. Let's say instead we only care about the titles of the movie and the description. Here is the query we'd use:

SELECT title, description FROM movies;

This will select the titles from movies table where the rating is greater than 4.

SELECT title FROM movies WHERE rating > 4;

You can also have more complex queries to get data. The following query finds all the movies with a rating greater than 4 and with a title of Cars.

SELECT title FROM movies WHERE rating > 4 AND title = 'Cars';

SQL also supports an OR statement. The following query will return any movie with a rating greater than 4, or any movies with the title Gigli. In other words, if a movie called Gigli is found with a rating equal to 1, it will still be returned along with any movie with a rating greater than 4.

SELECT title FROM movies WHERE rating > 4 OR title = 'Gigli';

Let's say that we just want a list of the best movies of all time. We can do a select statement that ensures ordering. The DESC keyword tells it to order the rating in descending order. ASC is the default.

SELECT title, rating FROM movies ORDER BY rating DESC;

Note: If no order by clause is specified, the database does not give any guarantees on what order your data will be returned in. At times it may seem like data you are getting back is in sorted order, but make sure not to rely on that in your code. Only rely on a sort if you also have an ORDER BY clause.

We've gotten a list of movies back, but it's way too long for our uses. Let's instead only get the top 5 movies that are returned using LIMIT:

SELECT title, rating FROM movies ORDER BY rating DESC LIMIT 5;

Exercise

Write a query on the movie table to return the worst movie of all time. There should be only 1 result returned. The result should include the title, description and rating of the movie.

Updating

The update statement is defined here in the postgres docs. It is used to change existing data in our database.

For example, if we do not think Gigli was actually that bad, and we want to change the rating to a 2, we can use an update statement:

UPDATE movies SET rating=2 WHERE title='Gigli';

Deleting

Deleting works similarly to a select statement. Here are the docs on delete

The statement below deletes the Dude Wheres My Car row from the database:

DELETE FROM movies WHERE title='Dude Wheres My Car';

We could also use compond statements here:

DELETE FROM movies WHERE id < 9 AND rating = 2;

ER Diagrams

Creating an ER diagram can be useful if you are designing a DB with lots of tables and relationships to one another. It may be useful to revist ER Diagrams after you have a firm understanding of databases. Here are some useful resources:

Wikipedia - ER Diagram
Ultimate Guide To ER Diagrams - Not so ultimate, but a good intro.

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