Book Image

Learn PostgreSQL - Second Edition

By : Luca Ferrari, Enrico Pirozzi
1 (2)
Book Image

Learn PostgreSQL - Second Edition

1 (2)
By: Luca Ferrari, Enrico Pirozzi

Overview of this book

The latest edition of this PostgreSQL book will help you to start using PostgreSQL from absolute scratch, helping you to quickly understand the internal workings of the database. With a structured approach and practical examples, go on a journey that covers the basics, from SQL statements and how to run server-side programs, to configuring, managing, securing, and optimizing database performance. This new edition will not only help you get to grips with all the recent changes within the PostgreSQL ecosystem but will also dig deeper into concepts like partitioning and replication with a fresh set of examples. The book is also equipped with Docker images for each chapter which makes the learning experience faster and easier. Starting with the absolute basics of databases, the book sails through to advanced concepts like window functions, logging, auditing, extending the database, configuration, partitioning, and replication. It will also help you seamlessly migrate your existing database system to PostgreSQL and contains a dedicated chapter on disaster recovery. Each chapter ends with practice questions to test your learning at regular intervals. By the end of this book, you will be able to install, configure, manage, and develop applications against a PostgreSQL database.
Table of Contents (22 chapters)
20
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21
Index

Aggregate functions

Aggregate functions perform a calculation on a set of rows and return a single row. PostgreSQL provides all the standard SQL aggregate functions:

  • AVG(): This function returns the average value.
  • COUNT(): This function returns the number of values.
  • MAX(): This function returns the maximum value.
  • MIN(): This function returns the minimum value.
  • SUM(): This function returns the sum of values.

Aggregate functions are used in conjunction with the group by clause. A group by clause splits a resultset into groups of rows and aggregate functions perform calculations on them. For example, if we wanted to count how many records there are for each category, PostgreSQL first groups the data and then counts it. The following diagram illustrates the process:

Chart  Description automatically generated

Figure 5.6: Group by aggregation

This diagram illustrates that PostgreSQL, before grouping the data, sorts it internally. Therefore, we must remember that a grouping...