Practical SQL Handbook, The: Using SQL Variants, Fourth Edition

Selecting Null Values

From earlier chapters (“NULLs” in Chapter 1), you may recall that NULL is a placeholder for unknown information. It does not mean zero or blank.

To clarify this NULL–zero difference, take a look at the following listing showing title and advance amount for books belonging to one particular publisher.

SQL

select title, advance
from titles
where pub_id = '0877'
title                                                         advance
============================================================= =======
Silicon Valley Gastronomic Treats                                0.00
Sushi, Anyone?                                                8000.00
Fifty Years in Buckingham Palace Kitchens                     4000.00
The Gourmet Microwave                                        15000.00
Onions, Leeks, and Garlic: Cooking Secrets of the
  Mediterranean                                               7000.00
The Psychology of Computer Cooking                             (NULL)
[6 rows]

A cursory perusal ...