🚀 The Missing SQL Basics Crash Course

Created September 28, 2021

Introduction

No matter if you are a DevOps/SysOps engineer, developer, or just a Linux enthusiast, you will most likely have to use SQL at some point in your career.

The guide is suitable for anyone working as a developer, system administrator, or a DevOps engineer and wants to learn the basics of SQL.

Databases

Before we dive deep into SQL, let's quickly define what a database is.

The definition of databases from Wikipedia is:

A database is an organized collection of data, generally stored and accessed electronically from a computer system.

In other words, a database is a collection of data stored and structured in different database tables.

Tables and columns

You've most likely worked with spreadsheet systems like Excel or Google Sheets. At the very basic, database tables are quite similar to spreadsheets.

Each table has different columns which could contain different types of data.

For example, if you have a todo list app, you would have a database, and in your database, you would have different tables storing different information like:

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The Users table will look like this:

+----+----------+---------------+--------+
| id | username | name          | active |
+----+----------+---------------+--------+
| 1  |    bobby | Bobby Iliev   |   true |
| 2  |   greisi | Greisi I.     |   true |
| 3  |  devdojo | Dev Dojo      |  false |
+----+----------+---------------+--------+

Rundown of the table structure:

Next we will learn how to install MySQL and create our first database.

MySQL

Now that you know what a database, table, and column are, the next thing that you would need to do is install a database service where you would be running your SQL queries on.

We would be using MySQL as it is free, open-source, and very widely used.

Installing MySQL

As we are going to use Ubuntu, in order to install MySQL run the following commands:

sudo apt update -y
sudo apt install mysql-server mysql-client

We are installing 2 packages, one is the actual MySQL server, and the other is the MySQL client, which would allow us to connect to the MySQL server and run our queries.

In order to check if MySQL is running, run the following command:

sudo systemctl status mysql.service

In order to secure your MySQL server, you could run the following command:

sudo mysql_secure_installation

Then follow the prompt and finally choose a secure password and save it in a secure place like a password manager.

With that, you would have MySQL installed on your Ubuntu server. The above should also work just fine on Debian.

Install MySQL on Mac

I would recommend installing MySQL using Homebrew:

brew install mysql

After that start MySQL:

brew services start mysql

And finally, secure it:

mysql_secure_installation

In case that you ever need to stop the MySQL service, you could do so with the following command:

brew services stop mysql

Install MySQL on Windows

In order to install MySQL on Windows, I would recommend following the steps from the official documentation here:

https://dev.mysql.com/doc/refman/8.0/en/windows-installation.html

Accessing MySQL via CLI

To access MySQL run the mysql command followed by your user:

mysql -u root -p

Creating a database

After that, switch to the demo database that we created in the previous chapter:

USE demo_db;

To exit the just type the following:

exit;

Configuring .my.cnf

By configuring the ~/.my.cnf file in your user's home directory, MySQL would allow you to login without prompting you for a password.

In order to make that change, what you need to do is first create a .my.cnf file in your user's home directory:

touch ~/.my.cnf

After that, set secure permissions so that other regular users could not read the file:

chmod 600 ~/.my.cnf

Then using your favorite text editor, open the file:

nano ~/.my.cnf

And add the following configuration:

[client]
user=YOUR_MYSQL_USERNAME
password=YOUR_MYSQL_PASSWORD

Make sure to update your MySQL credentials accordingly, then save the file and exit.

After that, if you run just mysql, you will be authenticated directly with the credentials that you've specified in the ~/.my.cnf file without being prompted for a password.

The mysqladmin command

As a quick test, you could check all of your open SQL connections by running the following command:

mysqladmin proc

The mysqladmin tool would also use the client details from the ~/.my.cnf file, and it would list your current MySQL process list.

Another cool thing that you could try doing is combining this with the watch command and kind of monitor your MySQL connections in almost real-time:

watch -n1 mysqladmin proc

To stop the watch command, just hit CTRL+C

GUI clients

If you prefer using GUI clients, you could take a look a the following ones and install them locally on your laptop:

This will allow you to connect to your database via a graphical interface rather than the mysql command-line tool.

Tables

Before we get started with SQL, let's learn how to create tables and columns.

As an example, we are going to create a users table with the following columns:

You need to specify the data type of each column.

In our case it would be like this:

Data types

The most common data types that you would come across are:

Let's have the following users table as an example:

For more information on all data types available, make sure to check out the official documentation here.

Creating a database

As we briefly covered in the previous chapter, before you could create tables, you would need to create a database by running the following:

mysql -u root -p
CREATE DATABASE demo_d;

Note: the database name needs to be unique, if you already have a database named demo_db you would receive an error that the database already exists.

You can consider this database as the container where we would create all of the tables in.

Once you've created the database, you need to switch to that database:

USE demo_db;

You can think of this as accessing a directory in Linux with the cd command. With USE, we switch to a specific database.

Alternatively, if you do not want to 'switch' to the specific database, you would need to specify the so-called fully qualified table name. For example, if you had a users table in the demo_db, and you wanted to select all of the entries from that table, you could use one of the following two approaches:

USE demo_db;
SELECT username FROM demo_db.users;
SELECT username FROM demo_db.users;

We are going to cover the SELECT statement more in-depth in the following chapters.

Creating tables

In order to create a table, you need to use the CREATE TABLE statement followed by the columns that you want to have in that table and their data type.

Let's say that we wanted to create a users table with the following columns:

The query that we would need to run to create that table would be:

CREATE TABLE users
(
    id INT,
    username VARCHAR(255),
    about TEXT,
    birthday DATE,
    active BOOL
);

Note: You need to select a database first with the USE command as mentioned above. Otherwise you will get the following error: ERROR 1046 (3D000): No database selected.

To list the available tables, you could run the following command:

SHOW TABLES;

Output:

+-------------------+
| Tables_in_demo_db |
+-------------------+
| users             |
+-------------------+

Dropping tables

You can drop or delete tables by using the DROP TABLE statement.

Let's test that and drop the table that we've just created:

DROP TABLE users;

The output that you would get would be:

Query OK, 0 rows affected (0.03 sec)

And now, if you were to run the SHOW TABLES; query again, you would get the following output:

Empty set (0.00 sec)

Allowing NULL values

By default, each column in your table can hold NULL values. In case that you don't wanted to allow NULL values for some of the columns in a specific table, you need to specify this during the table creation or later on change the table to allow that.

For example, let's say that we want the username column to be a required one, we would need to alter the table create statement and include NOT NULL right next to the username column like this:

CREATE TABLE users
(
    id INT,
    username VARCHAR(255) NOT NULL,
    about TEXT,
    birthday DATE,
    active BOOL
);

That way, when you try to add a new user, MySQL will let you know that the username column is required.

Specifying a primary key

The primary key column, which in our case is the id column, is a unique identifier for our users.

We want the id column to be unique, and also, whenever we add new users, we want the ID of the user to autoincrement for each new user.

This can be achieved with a primary key and AUTO_INCREMENT. The primary key column needs to be NOT NULL as well.

If we were to alter the table creation statement, it would look like this:

CREATE TABLE users
(
    id INT PRIMARY KEY AUTO_INCREMENT,
    username VARCHAR(255) NOT NULL,
    about TEXT,
    birthday DATE,
    active BOOL
);

Updating tables

In the above example, we created a new table and then dropped it as it was empty. However, in a real-life scenario, this would really be the case.

So whenever you need to add or remove a new column from a specific table, you would need to use the ALTER TABLE statement.

Let's say that we wanted to add an email column with type varchar to our users table.

The syntax would be:

ALTER TABLE users ADD email VARCHAR(255);

After that, if you were to describe the table, you would see the new column:

DESCRIBE users;

Output:

+----------+--------------+------+-----+---------+
| Field    | Type         | Null | Key | Default |
+----------+--------------+------+-----+---------+
| id       | int          | NO   | PRI | NULL    |
| username | varchar(255) | NO   |     | NULL    |
| about    | text         | YES  |     | NULL    |
| birthday | date         | YES  |     | NULL    |
| active   | tinyint(1)   | YES  |     | NULL    |
| email    | varchar(255) | YES  |     | NULL    |
+----------+--------------+------+-----+---------+

If you wanted to drop a specific column, the syntax would be:

ALTER TABLE table_name DROP COLUMN column_name;

Note: keep in mind that this is a permanent change, and if you have any critical data in the specific column, it would be deleted instantly.

You can use the ALTER TABLE statement to also change the data type of a specific column. For example, you could change the about column from TEXT to LONGTEXT type, which could hold longer strings.

Note: Important thing to keep in mind is that if a specific table already holds a particular type of data value like an integer, you can't alter it to varchar, for example. Only if the column does not contain any values, then you could make the change.

Basic Syntax

In this chapter, we will go over the basic SQL syntax.

SQL statements are basically the 'commands' that you run against a specific database. Through the SQL statements, you are telling MySQL what you want it to do, for example, if you wanted to get the username of all of your users stored in the users table, you would run the following SQL statement:

SELECT username FROM users ;

Rundown of the statement:

If you run the above statement, you will get no results as the new users table that we've just created is empty.

As a good practice, all SQL keywords should be with uppercase, however, it would work just fine if you use lower case as well.

Let's go ahead and cover the basic operations next.

INSERT

To add data to your database, you would use the INSERT statement.

Let's use the table that we created in the last chapter and insert 1 user into our users table:

INSERT INTO users(username, email, active) VALUES('bobby', '[email protected]', true);

Rundown of the insert statement:

SELECT

Once we've inserted that user, let's go ahead and retrieve the information.

To retrieve information from your database, you could use the SELECT statement:

SELECT * FROM users;

Output:

+----+----------+-------+----------+--------+---------------+
| id | username | about | birthday | active | email         |
+----+----------+-------+----------+--------+---------------+
|  1 | bobby    | NULL  | NULL     |      1 | [email protected] |
+----+----------+-------+----------+--------+---------------+

As we specify * right after the SELECT keyword, this means that we want to get all of the columns from the users table.

If we wanted to the only the username and the email columns instead, we would change the statement to:

SELECT username,email FROM users;

This will return all of the users, but as of the time being we have only 1:

+----------+----------------------+
| username | email                |
+----------+----------------------+
| bobby    | [email protected] |
+----------+----------------------+

UPDATE

In order to modify data in your database, you could use the UPDATE statement.

The syntax would look like this:

UPDATE users SET username='bobbyiliev' WHERE id=1;

Rundown of the statement:

NOTE: If we don't specify a WHERE clause, all of the entries inside the users table would be updated, and all users would have the username set to bobbyiliev. You need to be careful when you use the UPDATE statement without a WHERE clause, as every single row will be updated.

We are going to cover WHERE more in-depth in the next few chapters.

DELETE

As the name suggests, the DELETE statement would remove data from your database.

The syntax is as follows:

DELETE FROM users WHERE id=1;

Similar to the UPDATE statement, if you don't specify a WHERE clause, all of the entries from the table will be affected, meaning that all of your users will be deleted.

Comments

In case that you are writing a larger SQL script, it might be helpful to add some comments so that later on, when you come back to the script, you would know what each line does.

As with all programming languages, you can add comments in SQL as well.

There are two types of comments:

To do so, you just need to add -- before the text that you want to comment out:

SELECT * FROM users; -- Get all users

Similar to some other programming languages in order to comment multiple lines, you could wrap the text in /* */ as follows:

/*
Get all of the users
from your database
*/
SELECT * FROM users;

You could write that in .sql file and then run it later on, or execute the few lines directly.

Conclusion

Those were some of the most common basic SQL statements.

If you want to learn more about SQL make sure to check out my free eBook on GitHub:

Free Introduction to SQL eBook

To support me, you could follow me on Twitter: @bobbyiliev_.

If you already know the basics, I would suggest checking out Materialize. Materialize is a reactive database that delivers incremental view updates. We help developers easily build with streaming data using standard SQL.