An introduction to relational databases

Instructions

Fork, clone, and branch (training).

Please remember that demonstrations are to take notes, code alongs are to write code with the instructor, and practices are to write code with the support of the instructional team.

Objectives

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

Create a database table
Insert a row or rows into a database table
Retrieve a row or rows from a database table
Modify a database table after creation
Update a row or rows in a database table
Delete a row or rows from a database table

Prerequisites

A working PostgreSQL installation.
SQl Study

Introduction

Why are we talking about SQL?

Most applications need a data store to persist important information. A relational database is the most common datastore for a web application. SQL is the language of relational databases.

At it's simplest, a relational database is a mechanism to store and retrieve data in a tabular form.

Spreadsheets are a good analogy. Individual sheets as tables and the whole spreadsheet as a database. See this link for an example.

Why is this important?

Database tables are a good place to store key/value pairs, as long as the values are simple types (e.g. string, number). The keys are the column names and the values are stored in each row. That maps well to simple JSON objects. A group of rows from one table maps well to a JSON array.

What about more complicated data?

Database tables can reference other tables which allows arbitrary nesting of groups of simple types. This is something we'll be looking at more closely later.

Relational database management system (RDBMS)

A Database Server is a set of processes and files that manage the databases that store the tables. Sticking with our previous analogy a database server would map to Google Sheets.

Verb equivalence

CRUD (create, read, update and delete), SQL, HTTP, and Rails Controller action.

CRUD	SQL	HTTP	action
Create	INSERT	POST	create
Read	SELECT	GET	index/show
Update	UPDATE	PATCH	update
Delete	DELETE	DELETE	destroy

PostgreSQL

We'll be using PostgreSQL, a popular open source database server, which should already be installed on your computer.

On Macs you can run brew services list to see if PostgreSQL is running.

If the server isn't running, status not started, please start it using brew services start postgresql folder.

Create a database

Code along: CREATE DATABASE

We'll use sql-crud as the database to hold our tables and psql to interact with it. psql is PostgreSQL's command line client which lets us execute SQL commands interactively (REPL-like) and from scripts. It also has some built in commands we'll find useful.

$ psql sql-crud
psql: FATAL:  database "sql-crud" does not exist
$

But first we need to create the database. We'll use the CREATE DATABASE command from within psql. This is a SQL (Structure Query Language - see also the Wikipedia article) command and requires that we wrap the database name in double quotes (i.e. create database "sql-crud";). A - is not allowed as a name character in SQL unless the name is surrounded with double-quotes.

If we want to remove a database - be careful, this is unrecoverable - we use the DROP DATABASE command.

If we run psql without a parameter it will connect to our default database, usually named with our login.

psql

psql (9.4.5)
Type "help" for help.

and=> CREATE DATABASE "sql-crud";
CREATE DATABASE
and=>

Once we've created the database we can access it using the psql built-in command \c (for connect):

and=> \c sql-crud
You are now connected to database "sql-crud" as user "and".
sql-crud=>

Or we can access it from the command line using the psql command and passing the database name as an argument:

psql sql-crud

psql (9.4.5)
Type "help" for help.

sql-crud=>

psql has help for both its built-in commands and for SQL.

psql (9.4.5)
Type "help" for help.

sql-crud=> help
You are using psql, the command-line interface to PostgreSQL.
Type:  \copyright for distribution terms
       \h for help with SQL commands
       \? for help with psql commands
       \g or terminate with semicolon to execute query
       \q to quit
sql-crud=>

Let's look at some of the help for psql commands.

\l lists all the databases created on the server we're connected to.
\d (and its variations) shows information about the objects in the current database.
\i reads commands from a file

Now let's make sure we're in the right database:

sql-crud=> select current_catalog;
 current_database       
------------------
 sql-crud
(1 row)

sql-crud=>

We'll run all our SQL commands against the same database, so it's important that we consistently use sql-crud.

Creating tables

We create a table to define the names and types of data we want to store.

PostgreSQL's documentation is extensive and excellent, and we'll want to make use of it throughout the lesson.

Table basics - a brief overview of tables in an RDBMS.
Data Types - the data types available in PostgreSQL.
CREATE TABLE - detailed documentation of PostgreSQL's version of the SQL CREATE TABLE command.
DROP TABLE - detailed documentation of PostgreSQL's version of the SQL DROP TABLE command.

Note well, DROP TABLE is unrecoverable if it executes successfully.

Demonstration: CREATE TABLE

We'll create a table to hold people. By convention (the one we'll use throughout), tables are named with the pluralization of the name of the object whose data they hold. So, if each row is the data for a person, then the table is called people. By another convention, each table will have an id column that uniquely identifies each row. This unique id is the PRIMARY KEY of the table. We'll use the first line of data/people.csv for the rest of the column names.

What data-types should we use for each column?

We'll save the SQL statement to create the people table in create_table/people.sql. We can execute the commands in the file using psql -f <file> or from the psql prompt using \i <file>.

Code along: CREATE TABLE

Together, we'll create a table to hold information about cities. We'll use the first row of data/cities.csv for the column names.

What data-types should we use for each column?

We'll save the statement in create_table/cities.sql

Lab: CREATE TABLE

Create a table to hold information about pets. Use the first row of data/pets.csv for the names of the columns other than id. Use create_table/pets.sql to store the SQL statement.

Once you've created the table pets, create the table people from the demonstration.

Adding rows to a table

Inserting Data - overview of adding rows to a table.
INSERT - detailed documentation of PostgreSQL's version of the SQL INSERT INTO command.
COPY - detailed documentation of PostgreSQL's COPY command for loading data in bulk.

For inserting bulk data, PostgreSQL provides the COPY command. We won't use that command directly, as it executes relative to the server installation, rather we'll use psql's meta-command \copy allowing us to load data relative to where we run psql. Bulk loading is something available with most RDBMSs, but the specific commands and capabilities vary.

Demonstration: INSERT INTO and COPY

First we'll use variations of INSERT to add a few rows to people. We'll store the the commands in insert_into/people.sql.

Note that string literals in SQL statements are delimited by single quotes, i.e. '. To include a single quote in a string literal, double it, e.g. 'That''s interesting'. This is not an issue when loading from a valid CSV file using PostgreSQL's COPY command or psql's \copy command.

Next we'll load data in bulk from data/people.csv using \copy. We'll store that command in bulk_load/people.psql

Code along: INSERT INTO and COPY

Together we'll add a few rows to cities then we'll bulk load data/cities.csv.

Lab: INSERT INTO and COPY

Add a pet to the pets table using INSERT then bulk load data/pets.csv.

Next add a person to the people table using INSERT, then bulk load data/people.csv.

Retrieving rows from a table

This is about the query part of Structured Query Language. Query statements can run from almost trivial to highly complex. They provide a mechanism to retrieve and summarize the data in your database.

Queries - TOC of the Queries section of PostgreSQL's documentation for The SQL Language.
SELECT - detailed documentation of PostgreSQL's version of the SQL SELECT command.

Demonstration: SELECT

Let's see some what we can learn about the people in the database.

Code along: SELECT

Together we'll build a query to get the count of cities by country.

Lab: SELECT

Write a query to get the count of animals by kind born before 2010.

Then write a query to count people by height.

Changing the structure of a table

Modifying Tables - overview of changing tables.
ALTER TABLE - detailed documentation of PostgreSQL's version of the SQL ALTER TABLE command.

Demonstration: ALTER TABLE

We'll remove the column weight from people.

Code along: ALTER TABLE

We'll change the type of the columns longitude and latitude in cities.

Lab: ALTER TABLE

Add the column weight to pets then remove the column weight from people.

Changing the data in rows in a table

Updating Data - overview of changing rows
UPDATE - detailed documentation of PostgreSQL's version of the SQL UPDATE command.

Demonstration: UPDATE

We'll update some people's height.

Code along: UPDATE

Let's update some cities population.

Lab: UPDATE

Update weight for pets then height for people.

Removing rows from a table

Deleting Data - overview of removing rows from a table
DELETE - detailed documentation of PostgreSQL's version of the SQL DELETE command.
TRUNCATE - detailed documentation of PostgreSQL's TRUNCATE command.

Demonstration: DELETE

We'll remove a few people from the database.

Note, TRUNCATE <table name>; is functionally equivalent to DELETE FROM <table name>;, it will remove all the rows from the table.

byau/sql-crud