JOIN (Association) SQL Queries

Follow along with code examples here!

So, what makes Postgres a "relational" database management system? Relationships!

In this lesson, we'll learn how we can establish relationships between tables using primary and foreign keys and how to manipulate related tables with SQL.

Table of Contents

• Terms

• How should I create my database?

• Two Tables and Foreign Keys

• Association SQL Queries

  • Setup

  • Some Questions To Answer

• Entity Relation Diagrams and Many To Many Relationships

Terms

• Schema Design - the process of planning the structure and relationships of the tables in a database.

• Primary Key - a column in a table that uniquely identifies each row in the table.

• Foreign Key - a column in a table that references the primary key of another table.

• One-to-Many - a relationship between two tables in which instances in one table can be referenced by many instances in another table.

• JOIN - A SQL statement that combines the columns of two tables.

• Entity Relationship Diagram - a diagram that illustrates the relationships between tables.

• Many-to-Many - a relationship between two tables in which the instances of each table can be referenced by many instances in the other table.

• Association/Junction Table - a table used to create a many-to-many relationship using two foreign keys to reference two tables.

How should I create my database?

Imagine that you are building a social network app for people and their pets. Users can sign up and add pets to their accounts.

To account for all of the data, you might make a single table that stores each instance of a user-pet relationship:

SQL:

CREATE TABLE all_data (
	id SERIAL PRIMARY KEY,
	owner_name TEXT,
	pet_name TEXT,
	type TEXT
)

all_data Table:

id	owner_name	pet_name	type
1	Ann Duong	Bora	bird
2	Ann Duong	Tora	dog
3	Ann Duong	Kora	dog
4	Ben Spector
5	Reuben Ogbonna	Juan Pablo	dog
6	Reuben Ogbonna	Pon Juablo	cat
7	Carmen Salas	Khalo	dog
8	Carmen Salas	Frida	cat

Q: What are the issues with storing the data in this way?

• There is a lot of duplicate data in the owner_name column

Two Tables and Foreign Keys

Instead of storing all data in a single table, we can break up the table into two related tables:

1. A people table to store data about people

2. A pets table to store data about pets

   • The pets table has an owner_id column that references the primary key the people table.

   • The pets.owner_id column is known as a foreign key.

CREATE TABLE people (
	id SERIAL PRIMARY KEY,
	name TEXT NOT NULL
);
CREATE TABLE pets (
	id SERIAL PRIMARY KEY,
	name TEXT NOT NULL, 
	type TEXT NOT NULL, 
	owner_id INTEGER REFERENCES people --foreign key
);

people Table:

id	name
1	Ann Duong
2	Reuben Ogbonna
3	Carmen Salas
4	Ben Spector

pets Table:

id	name	type	owner_id
1	Khalo	dog	3
2	Juan Pablo	dog	2
3	Bora	bird	1
4	Frida	cat	3
5	Tora	dog	1
6	Pon Juablo	cat	2
7	Kora	dog	1

Q: What are the tradeoffs of this schema design?

We no longer have duplicate data It is not exactly clear anymore the name of the person who owns each pet

With these two tables, a one-to-many relationship has been formed: a person can have many pets. This is one of the most common relationships between tables in a database. Some other examples include:

• A user has many posts

• A restaurant has many reviews

• An album has many songs

• Can you think of any more?

Association SQL Queries

Setup

Use the following commands to quickly create the people and pets tables in a database of your choosing.

DROP TABLE IF EXISTS pets; -- do this first because pets depends on people
DROP TABLE IF EXISTS people;

CREATE TABLE people (
	id SERIAL PRIMARY KEY,
	name TEXT NOT NULL
);

INSERT INTO people (name) VALUES ('Ann Duong');
INSERT INTO people (name) VALUES ('Reuben Ogbonna');
INSERT INTO people (name) VALUES ('Carmen Salas');
INSERT INTO people (name) VALUES ('Ben Spector');

CREATE TABLE pets (
	id SERIAL PRIMARY KEY,
	name TEXT NOT NULL, 
	type TEXT NOT NULL, 
	owner_id INTEGER REFERENCES people
);

INSERT INTO pets (name, type, owner_id) VALUES ('Khalo', 'dog', 3);
INSERT INTO pets (name, type, owner_id) VALUES ('Juan Pablo', 'dog', 2);
INSERT INTO pets (name, type, owner_id) VALUES ('Bora', 'bird', 1);
INSERT INTO pets (name, type, owner_id) VALUES ('Tora', 'dog', 1);
INSERT INTO pets (name, type, owner_id) VALUES ('Frida', 'cat', 3);
INSERT INTO pets (name, type, owner_id) VALUES ('Pon Juablo', 'cat', 2);
INSERT INTO pets (name, type, owner_id) VALUES ('Kora', 'dog', 1);

• What is the primary key for each table?

• Are there any foreign keys?

• How is the relationship between the two tables created using SQL?

Some Questions To Answer

Now that we're set up, let's answer some questions!

We'll start with a few simple ones. What SQL statements could we write to find out:

• How many people are in our database?

• What are the names of the cats in our database?

• How many birds and dogs are in our database?

Answers

SELECT COUNT(*)
FROM people;

SELECT name
FROM pets
WHERE type = 'cat';

SELECT COUNT(*)
FROM pets
WHERE type = 'dog' OR type = 'bird';

Now let's try a harder one: What are the names of all the pets owned by Ann?

In order to answer this question, it will be helpful to use a JOIN

SELECT * 
FROM people 
	JOIN pets ON people.id = pets.owner_id;

person.id	name	pet.id	name	type	owner_id
1	Ann Duong	3	Bora	bird	1
1	Ann Duong	4	Tora	dog	1
1	Ann Duong	7	Kora	dog	1
2	Reuben Ogbonna	2	Juan Pablo	dog	2
2	Reuben Ogbonna	6	Pon Juablo	cat	2
3	Carmen Salas	1	Khalo	dog	3
3	Maya Salas	5	Frida	cat	3

We can modify our SELECT and add a WHERE clause to answer our question!

SELECT pets.name 
FROM people 
	JOIN pets ON people.id = pets.owner_id
WHERE people.name = 'Ann Duong';

Let's see if we can answer some other questions:

• Who is the owner of Frida?

• What are the names of the people who own cats?

• How many pets does Carmen have?

Answers

SELECT people.name
FROM people 
	JOIN pets ON people.id = pets.owner_id
WHERE pets.name = 'Frida';

SELECT people.name
FROM people 
	JOIN pets ON people.id = pets.owner_id
WHERE pets.type = 'cat';

SELECT COUNT(*)
FROM people 
	JOIN pets ON people.id = pets.owner_id
WHERE	people.name = 'Carmen Salas';

Entity Relation Diagrams and Many To Many Relationships

We can illustrate the relationships between tables with an entity relation diagram (ERD):

ERD with one to many and many to many relationships

created using https://dbdiagram.io/

Above, we've introduced a many-to-many relationship that is created using three tables: A student can be enrolled in many classes and a class can enroll many students.

The enrollments table in the middle is called an association/junction table because its records represent the association of two separate entities. An association/junction table can consist entirely of foreign keys.

Q: Can you think of other many-to-many relationships in the apps you use?