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Learn PostgreSQL

You're reading from   Learn PostgreSQL Use, manage, and build secure and scalable databases with PostgreSQL 16

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Product type Paperback
Published in Oct 2023
Publisher Packt
ISBN-13 9781837635641
Length 744 pages
Edition 2nd Edition
Languages
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Authors (2):
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Luca Ferrari Luca Ferrari
Author Profile Icon Luca Ferrari
Luca Ferrari
Enrico Pirozzi Enrico Pirozzi
Author Profile Icon Enrico Pirozzi
Enrico Pirozzi
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Toc

Table of Contents (22) Chapters Close

Preface 1. Introduction to PostgreSQL 2. Getting to Know Your Cluster FREE CHAPTER 3. Managing Users and Connections 4. Basic Statements 5. Advanced Statements 6. Window Functions 7. Server-Side Programming 8. Triggers and Rules 9. Partitioning 10. Users, Roles, and Database Security 11. Transactions, MVCC, WALs, and Checkpoints 12. Extending the Database – the Extension Ecosystem 13. Query Tuning, Indexes, and Performance Optimization 14. Logging and Auditing 15. Backup and Restore 16. Configuration and Monitoring 17. Physical Replication 18. Logical Replication 19. Useful Tools and Extensions 20. Other Books You May Enjoy
21. Index

Using distinct

We will now discuss another kind of query: the distinct query. Firstly, however, we need to introduce another very useful function for the DBA called the coalesce function. The coalesce function, given two or more parameters, returns the first value that is not NULL.

For example, let’s use the coalesce function for the test value:

forumdb=> select coalesce(NULL,'test');
 coalesce
----------
 test
(1 row)

In the preceding query, the coalesce function returns test because the first argument is NULL and the second argument is not NULL.

Now, let’s insert a new category:

forumdb=> insert into categories (title) values ('New Category');
INSERT 0 1

And then let’s perform the following query:

forumdb=# \pset null (NULL)
Null display is "(NULL)".
forumdb=> select pk,title,description from categories;
 pk |         title         |           description      
----+-----------------------...
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