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Firebird Null Guide

NULL behaviour and pitfalls in Firebird SQL

Paul Vinkenoog

------ 2007 – Document version 1.x

Table of Contents

What is NULL?
NULL support in Firebird SQL
NULL in operations
Internal functions and directives
Aggregate functions
Conditional statements and loops
Keys and unique indices
CHECK constraints
User-Defined Functions (UDFs)
Converting to and from NULL
Altering populated tables
Changing the nullability of a domain
Testing for NULL and equality in practice
A. NULL-related bugs in Firebird
Bugs that crash the server
Other bugs
B. Document history
C. License notice
Alphabetical index

What is NULL?

Time and again, support questions pop up on the Firebird mailing lists about “strange things” happening with NULLs. The concept seems difficult to grasp – perhaps partly because of the name, which suggests a kind of “nothing” that won't do any harm if you add it to a number or stick it to the back of a string. In reality, performing such operations will render the entire expression NULL.

This guide explores the behaviour of NULL in Firebird SQL, points out common pitfalls and shows you how to deal safely with expressions that contain NULL or may resolve to NULL.

If you only need a quick reference to refresh your memory, go to the summary at the end of the guide.

So – what is it?

In SQL, NULL is not a value. It is a state indicating that an item's value is unknown or nonexistent. It is not zero or blank or an “empty string” and it does not behave like any of these values. Few things in SQL lead to more confusion than NULL, and yet its workings shouldn't be hard to understand as long as you stick to the following simple definition: NULL means unknown.

Let me repeat that:


Keep this line in mind as you read through the rest of the guide, and most of the seemingly illogical results you can get with NULL will practically explain themselves.


A few sentences and examples in this guide were taken from the Firebird Quick Start Guide, first published by IBPhoenix, now part of the Firebird Project.

NULL as a default state

Because NULL means “value unknown”, it is the logical default state for any field or variable that has been created but not provided with a value:

  • If you declare a variable in a stored procedure or trigger, its value is undefined and its state is NULL from the moment of creation until some value is assigned to it. The same is true for output parameters in stored procedures.

  • If you insert a record into a table and you only provide values for part of the fields, the remaining fields will be initialised to NULL except where a default value is in effect or a value is assigned by a “before insert” trigger.

  • If you add a column to a table that already has records, the fields added to the existing records will be NULL, except if you declare the column as NOT NULL and specify a default value for it. Note that both conditions must be satisfied for the fields to become anything other than NULL.

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