null ("M" Technical Reference)

[This content is no longer valid. For the latest information on "M", "Quadrant", SQL Server Modeling Services, and the Repository, see the Model Citizen blog.]

null is a distinguished value of type Null that serves as a place holder for some other value that is not known. The value null can be added to the value space of any non-collection type by using the postfix operator ?. The result is a nullable type, very similar to the nullable types in the T-SQL language. The following example makes true assertions about some nullable types.

null in Integer32?
null in Text?
!(null in Text)

Arithmetic operations on a null operand return null as seen in the following code.

1 + null == null
null * 3 == null

Logical operators, conditional, and constraints require non-nullable operands.

A null coalescing expression of the form a ?? b requires a to be nullable. If a is not null, the result of a ?? b is a; otherwise, the result is b. The operation evaluates b only if a is null. b must be of the same type a without the value null.

Syntax

null

By setting a value equal to this literal, you are specifying that its value is unknown.

Examples

The following Person entity initializes the MiddleName field to null.

module Contacts
{
    type Person
    {
        FirstName : Text;
        MiddleName : Text? = null;
        LastName : Text;
    }
}

The values in the following extent conform to the Person type.

    People : Person*;
    People 
    {
        {FirstName => "Joe", LastName => "Smith"},
        {FirstName => "Jim", MiddleName => null, LastName => "Smith"},
        {FirstName => "John", MiddleName => "Bob", LastName => "Smith"}
    }