In Chapter 1, Understanding and Describing Data, we discussed analytics and how we can use data to obtain valuable information. While we could, in theory, analyze all data by hand, computers are far better at the task and are certainly the preferred tool for storing, organizing, and processing data. Among the most critical of these data tools is the relational database and the language used to access it, Structured Query Language (SQL). These two technologies have been cornerstones of data processing and continue to be the data backbone of most companies that deal with substantial amounts of data.

Companies use SQL as the primary method for storing much of their data. Furthermore, companies now take much of this data and put it into specialized databases called data warehouses and data lakes so that they can perform advanced analytics on their data. Virtually all of these data warehouses and data lakes are accessed using SQL. We'll be looking at working with SQL...

A relational database is a database that utilizes the relational model of data. The relational model, invented by Edgar F. Codd in 1970, organizes data as relations, or sets of tuples. Each tuple consists of a series of attributes, which generally describe the tuple. For example, we could imagine a customer relation, where each tuple represents a customer. Each tuple would then have attributes describing a single customer, giving information such as first name, last name, and age, perhaps in the format (John, Smith, 27). One or more of the attributes is used to uniquely identify a tuple in a relation and is called the relational key. The relational model then allows logical operations to be performed between relations.

In a relational database, relations are usually implemented as tables, as in an Excel spreadsheet. Each row of the table is a tuple, and the attributes are represented as columns of the table. While not technically required, most tables...

As previously mentioned, each column in a table has a data type. We review the major data types here.

Numeric

Numeric data types are data types that represent numbers. The following diagram provides an overview of some of the major types:

Figure 2.1: Major numeric data types

Character

Character data types store text information. The following diagram summarizes the character data types:

Figure 2.2: Major character data types

Under the hood, all of the character data types use the same underlying data structure in PostgreSQL and many other SQL databases, and most modern developers do not use char(n).

Boolean

Booleans are a data type used to represent True or False. The following table summarizes values that are represented as a Boolean when used in a query with a Boolean data column type:

Figure 2.3: Accepted Boolean values

While all of these values are accepted, the...

The most common operation in a database is reading data from a database. This is almost exclusively done through the use of the SELECT keyword.

Basic Anatomy and Working of a SELECT Query

Generally speaking, a query can be broken down into five parts:

• Operation: The first part of a query describes what is going to be done. In this case, this is the word SELECT, followed by the names of columns combined with functions.
• Data: The next part of the query is the data, which is the FROM keyword followed by one or more tables connected together with reserved keywords indicating what data should be scanned for filtering, selection, and calculation.
• Conditional: A part of the query that filters the data to only rows that meet a condition usually indicated with WHERE.
• Grouping: A special clause that takes the rows of a data source, assembles them together using a key created by a GROUP BY clause, and then calculates a value using...

Now that we know how to read data from tables, we will now look at how to create new tables. There are fundamentally two ways to create tables: creating blank tables or using SELECT queries.

Creating Blank Tables

To create a new blank table, we use the CREATE TABLE statement. This statement takes the following structure:

CREATE TABLE {table_name} (
{column_name_1} {data_type_1} {column_constraint_1},
{column_name_2} {data_type_2} {column_constraint_2},
{column_name_3} {data_type_3} {column_constraint_3},
…
{column_name_last} {data_type_last} {column_constraint_last},
);

Here {table_name} is the name of the table, {column_name} is the name of the column, {data_type} is the data type of the column, and {column_constraint} is one or more optional keywords giving special properties to the column. Before we discuss how to use the CREATE TABLE query, we will first discuss column constraints.

Column Constraints

Column constraints are keywords that...

Over time, you may also need to modify a table by adding columns, adding new data, or updating existing rows. We will discuss how to do that in this section.

Adding and Removing Columns

To add new columns to an existing table, we use the ADD COLUMN statement as in the following query:

ALTER TABLE {table_name}
ADD COLUMN {column_name} {data_type};

Let's say, for example, that we wanted to add a new column to the products table that we will use to store the products' weight in kilograms called weight. We could do this by using the following query:

ALTER TABLE products
ADD COLUMN weight INT;

This query will make a new column called weight in the products table and will give it the integer data type so that only numbers can be stored within it.

If you want to remove a column from a table, you can use the DROP column statement:

ALTER TABLE {table_name}
DROP COLUMN {column_name};

Here, {table_name} is the name of the table you want to...

We often discover that data in a table is incorrect, and therefore can no longer be used. At such times, we need to delete data from a table.

Deleting Values from a Row

Often, we will be interested in deleting a value in a row. The easiest way to accomplish this task is to use the UPDATE structure we already discussed and to set the column value to NULL like so:

UPDATE {table_name}
SET {column_1} = NULL,
    {column_2} = NULL,
    ...
    {column_last} = NULL
WHERE
 {conditional};

Here, {table_name} is the name of the table with the data that needs to be changed, {column_1}, {column_2},… {column_last} is the columns whose values you want to delete, and {WHERE} is a conditional statement like one you would find in a SQL query.

Let's say, for instance, that we have the wrong email on file for the customer with the customer ID equal to 3. To fix that, we can use the following...

In this chapter, we went through the basics of SQL, tables, and queries. You may be wondering, then, what SQL has to do with analytics. You may have seen some parallels between the first two chapters. When we talk about a SQL table, it should be clear that it can be thought of as a dataset. Rows can be considered individual units of observation and columns can be considered features. If we view SQL tables in this way, we can see that SQL is a natural way to store datasets in a computer.

However, SQL can go further than just providing a convenient way to store datasets. Modern SQL implementations also provide tools for processing and analyzing data through various functions. Using SQL, we can clean data, transform data to more useful formats, and analyze data with statistics to find interesting patterns. The rest of this book will be dedicated to understanding how SQL can be used for these purposes productively and efficiently.

Relational databases are a mature and ubiquitous technology that is used to store and query data. Relational databases store data in the form of relations, also known as tables, which allow for an excellent combination of performance, efficiency, and ease of use. SQL is the language used to access relational databases. SQL is a declarative language that allows users to focus on what to create, as opposed to how to create it. SQL supports many different data types, including numeric data, text data, and even data structures.

When querying data, SQL allows a user to pick which fields to pull, as well as how to filter the data. This data can also be ordered, and SQL allows for as much or as little data as we need to be pulled. Creating, updating, and deleting data is also fairly simple and can be quite surgical.

Now that we have reviewed the basics of SQL, we will discuss how SQL can be used to perform the first step in data analytics, cleaning, and the transformation of...