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Using NULL in Criteria

Written by Andrew Gould

In this tutorial

Testing Whether Something Is Null
Testing Whether Something Is Not Null
Using Equals with NULL

In SQL Server when a value has not been entered in a column for a particular record it is referred to as NULL. There is a unique syntax for writing criteria to find these null entries.

You can spot nulls in a set of results quite easily!

Testing Whether Something Is Null

You can find records which contain nulls by asking for those rows where a particular field IS NULL:

SELECT

FilmName

,FilmBoxOfficeDollars

FROM

tblFilm

WHERE

FilmBoxOfficeDollars IS NULL

Simple! The results of this query are shown below:

These films have no information in the FilmBoxOfficeDollars field.

Testing Whether Something Is Not Null

Testing if a field is not null uses a syntax which is almost as simple as the one shown above:

SELECT

FilmName

,FilmBoxOfficeDollars

FROM

tblFilm

WHERE

FilmBoxOfficeDollars IS NOT NULL

The above query finds all of the records which have a value in the FilmBoxOfficeDollars field.

All of these records have some sort of value in the box office column.

Using Equals with NULL

Occasionally, people who are new to SQL will try to use the equals sign when testing for null, as in the example shown below:

SELECT

FilmName

,FilmBoxOfficeDollars

FROM

tblFilm

WHERE

FilmBoxOfficeDollars = NULL

With SQL Server's default settings this query won't return any results, even if there are nulls in the FilmBoxOfficeDollars field. It is possible to change this behaviour, however, by adding an extra line to the top of your query:

--Change the behaviour of NULL

SET ANSI_NULLS OFF

SELECT

FilmName

,FilmBoxOfficeDollars

FROM

tblFilm

WHERE

FilmBoxOfficeDollars = NULL

The query will now successfully find records with null in the FilmBoxOfficeDollars field, as shown below:

The query now finds nulls in the specified field.

It's difficult to think of a good reason for changing the standard behaviour of nulls in criteria. In fact, according to the Microsoft website, future versions of SQL Server will no longer allow you to do this.

That's everything on basic criteria in SQL queries!

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