HoldenMills / sql-join

An Introduction to PostgreSQL Foreign Keys

An Introduction to PostgreSQL Foreign Keys

Instructions

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Objectives

By the end of this lesson, students should be able to:

• Add a foreign key reference to an existing table.
• Create a table with a foreign key reference.
• Insert a row which includes a reference to the id of a row in another table.
• Update a row setting a reference to the id of a row in another table.
• Retrieve rows from two tables using a JOIN condition

Prerequisites

• An introduction to relational databases

Introduction

We've been talking about relational databases but haven't yet specified what constitutes a relationship.

In an RDBMS, relationships between tables are specified with a FOREIGN KEY constraint on a column in one table that REFERENCES the PRIMARY KEY constraint on the id column of another table. An INDEX is usually added to the foreign key column to speed access to matched rows.

This reference and index tells the RDBMS how you intend to use related tables.

There are a number of standard cardinalities for relationships: 1-to-1, 1-to-(0 or 1), 1-to-(0 or more), and (0 or more)-to-(0 or more). The last two are more frequently called 1-to-many and many-to-many. We'll look at this last type of relationship in a following lesson.

Create a database

Code along

We'll use sql-join as the working database. We'll create it using the CREATE DATABASE command with sql-crud specified as the template.

$ psql

psql (9.4.5)
Type "help" for help.

and=# CREATE DATABASE "sql-join" TEMPLATE "sql-crud";
CREATE DATABASE
and=# \c sql-join
You are now connected to database "sql-join" as user "and".
sql-join=#

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Create a foreign key

We'll use the convention that a foreign key should be named for the singular of the table referenced with the column referenced appended after an underscore. So, if we're adding a reference to the cities table and its id column we'll create a column called city_id. This convention should not be followed when there is a semantically superior name available.

• Constraints - An overview of the variety of constraints that PostgreSQL provides.
• CREATE TABLE - detailed documentation of PostgreSQL's version of the SQL CREATE TABLE command.
• ALTER TABLE - detailed documentation of PostgreSQL's version of the SQL ALTER TABLE command.
• Index Introduction - The introductory section of the chapter on indexes in PostgreSQL.
• CREATE INDEX - detailed documentation of PostgreSQL's version of the SQL CREATE INDEX command.

Demonstration

We'll add a place of birth to people which references a city in the city table. We'll use place_of_birth_id as the column name as this is a more semantic name for the column.

Code along

We'll create an addresses table which references the cities table. Then we'll add a reference from the people table to the addresses table.

Practice

We'll add an owner reference to the pets table.

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Creating explicit relationships between rows in different tables.

We'll update or insert rows with appropriate values. Note that a foreign key constraint will disallow invalid values in the referencing column.

• UPDATE - detailed documentation of PostgreSQL's version of the SQL UPDATE command.
• INSERT - detailed documentation of PostgreSQL's version of the SQL INSERT INTO command.

Demonstration

We'll set the place of birth for some people.

Code along

We'll insert an address associated with Somerville. Then we'll bulk load addresses and connect some of them to cities. After that, we'll set some people's current addresses.

Practice

We'll set the owner reference for some pets. Pick at least two people to be folks with too many pets.

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Retrieving rows from two related tables

The join allows queries to return associated data from two tables as a single row.

• Joins Between Tables - An introduction to querying multiple tables
• SELECT - detailed documentation of PostgreSQL's version of the SQL SELECT command.

Demonstration

We'll count all the people by place of birth. Then we'll look at information about people born in a particular city or cities.

Code along

We'll query people, the associated addresses, and the cities associated with those addresses.

Practice

Let's check for hoarders. That's anyone with more than 3 dogs, 4 birds, 5, cats, or 20 fish.

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