Introduction

Structured Query Language or SQL is the standard language for interacting with relational databases. It is used to create, read, update, and delete data from a database. SQL is a declarative language, meaning that you tell the database what you want to do, and it figures out how to do it.

first_name	last_name	age
John	Doe	30
Jane	Doe	28
Bob	Smith	35

Using SQL, we are able to define the schema (structure) of the table and then interact with it.

Databases are similar to spreadsheets in that they are made up of tables. Each table has a name and a schema. The schema defines the columns of the table and the type of data that each column can contain. For example, a table for users might have the following schema:

Column	Type
first_name	varchar
last_name	varchar
age	int

The first_name and last_name columns are of type varchar which means they can contain any string. The age column is of type int which means it can contain any integer. A full list of data types can be found at W3Schools.

Basics

SQL is made up of statements. Each statement is made up of keywords and clauses. Keywords are reserved words that have special meaning in SQL. Clauses are the parameters that you pass to the keywords. For example, the following statement selects all columns from the users table:

SELECT * FROM users;

Will return:

first_name	last_name	age
John	Doe	30
Jane	Doe	28
Bob	Smith	35

The SELECT keyword tells the database that we want to select data from the database. The * is a wildcard that tells the database that we want to select all columns. The FROM keyword tells the database which table we want to select data from. In this case, we are selecting data from the users table.

Clauses

Clauses are the parameters that you pass to the keywords. For example, the WHERE clause is used to filter data. The following statement selects all columns from the users table where the age column is equal to 30:

SELECT * FROM users WHERE age = 30;

Will return:

first_name	last_name	age
John	Doe	30

The WHERE clause tells the database that we want to filter the data. The age = 30 clause tells the database that we want to filter the data where the age column is equal to 30.

Operators

Operators are used to compare values. The following operators are supported by most databases:

Operator	Description
=	Equal
<>	Not Equal
>	Greater Than
<	Less Than
>=	Greater Than or Equal
<=	Less Than or Equal

This means we can use the following operators in our WHERE clause:

SELECT * FROM users WHERE age >= 30;

Will return:

first_name	last_name	age
Bob	Smith	35
John	Doe	30