Book Image

PostgreSQL Development Essentials

By : Manpreet Kaur, Baji Shaik
Book Image

PostgreSQL Development Essentials

By: Manpreet Kaur, Baji Shaik

Overview of this book

PostgreSQL is the most advanced open source database in the world. It is easy to install, configure, and maintain by following the documentation; however, it’s difficult to develop applications using programming languages and design databases accordingly. This book is what you need to get the most out of PostgreSQL You will begin with advanced SQL topics such as views, materialized views, and cursors, and learn about performing data type conversions. You will then perform trigger operations and use trigger functions in PostgreSQL. Next we walk through data modeling, normalization concepts, and the effect of transactions and locking on the database. The next half of the book covers the types of indexes, constrains, and the concepts of table partitioning, as well as the different mechanisms and approaches available to write efficient queries or code. Later, we explore PostgreSQL Extensions and Large Object Support in PostgreSQL. Finally, you will perform database operations in PostgreSQL using PHP and Java. By the end of this book, you will have mastered all the aspects of PostgreSQL development. You will be able to build efficient enterprise-grade applications with PostgreSQL by making use of these concepts
Table of Contents (17 chapters)
PostgreSQL Development Essentials
About the Authors
About the Reviewers

Using the HAVING clause

In the previous section, we discussed about GROUP BY clause, however if you want to restrict the groups of returned rows, you can use HAVING clause. The HAVING clause is used to specify which individual group(s) is to be displayed, or in simple language we use the HAVING clause in order to filter the groups on the basis of an aggregate function condition.

Note: The WHERE clause cannot be used to return the desired groups. The WHERE clause is only used to restrict individual rows. When the GROUP BY clause is not used, the HAVING clause works like the WHERE clause.

The syntax for the PostgreSQL HAVING clause is as follows:

SELECT expression1, expression2, ... expression_n, 
aggregate_function (expression)
FROM tables
WHERE conditions
GROUP BY expression1, expression2, ... expression_n
HAVING group_condition;

Parameters or arguments

aggregate_function can be a function such as SUM, COUNT, MIN, MAX, or AVG.

expression1, expression2, ... expression_n are expressions that are not encapsulated within an aggregate function and must be included in the GROUP BY clause.

conditions are the conditions used to restrict the groups of returned rows. Only those groups whose condition evaluates to true will be included in the result set.

Let's consider an example where you try to fetch the product that has sales>10000:

SELECT product, SUM(sale) AS "Total sales"
FROM order_details
GROUP BY product
Having sum(sales)>10000;

The PostgreSQL HAVING clause will filter the results so that only the total sales greater than 10000 will be returned.