NULL Values

NULL values represent missing, unknown, or not-applicable values. For example, let's say that you want to add a membership_expiration_date to the customers table. Some customers might be permanent memberstheir memberships will never expire. For those customers, the membership_expiration_date is not applicable and should be set to NULL. You may also find some customers who don't want to provide you with their birth dates. The birth_date column for these customers should be NULL.

In one case, NULL means not applicable. In the other case, NULL means don't know. A NULL membership_expiration_date does not mean that you don't know the expiration date, it means that the expiration date does not apply. A NULL birth_date does not mean that the customer was never born(!); it means that the date of birth is unknown.

Of course, when you create a table, you can specify that a given column cannot hold NULL values (NOT NULL). When you do so, you aren't affecting the data type of the column; you're just saying that NULL is not a legal value for that particular column. A column that prohibits NULL values is mandatory; a column that allows NULL values is optional.

You may be wondering how a data type could hold all values legal for that type, plus one more value. The answer is that PostgreSQL knows whether a given column is NULL not by looking at the column itself, but by first examining a NULL indicator (a single bit) stored separately from the column. If the NULL indicator for a given row/column is set to TRUE, the data stored in the row/column is meaningless. This means that a data row is composed of values for each column plus an array of indicator bitsone bit for each optional column.

Part I: General PostgreSQL Use

Introduction to PostgreSQL and SQL

Working with Data in PostgreSQL

PostgreSQL SQL Syntax and Use


Part II: Programming with PostgreSQL

Introduction to PostgreSQL Programming

Extending PostgreSQL


The PostgreSQL C APIlibpq

A Simpler C APIlibpgeasy

The New PostgreSQL C++ APIlibpqxx

Embedding SQL Commands in C Programsecpg

Using PostgreSQL from an ODBC Client Application

Using PostgreSQL from a Java Client Application

Using PostgreSQL with Perl

Using PostgreSQL with PHP

Using PostgreSQL with Tcl and Tcl/Tk

Using PostgreSQL with Python

Npgsql: The .NET Data Provider

Other Useful Programming Tools

Part III: PostgreSQL Administration

Introduction to PostgreSQL Administration

PostgreSQL Administration

Internationalization and Localization


Replicating PostgreSQL Data with Slony

Contributed Modules


PostgreSQL(c) The comprehensive guide to building, programming, and administering PostgreSQL databases
PostgreSQL(c) The comprehensive guide to building, programming, and administering PostgreSQL databases
ISBN: 735712573
Year: 2004
Pages: 261 © 2008-2020.
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