Function Reference > Constants > NULL
  

NULL

Indicates that a value is either unknown or undefined. NULL is not equivalent to a blank or empty string (for character columns) or 0 (for numerical columns).
Although you can write expressions that return nulls, any column that has the NOT NULL or PRIMARY KEY constraint will not accept nulls. Therefore, if Data Integration tries to write a null value to a column with one of these constraints, the database will reject the row and Data Integration will write it to the reject file. Be sure to consider nulls when you create transformations.
Functions can handle nulls differently. If you pass a null value to a function, it might return 0 or NULL, or it might ignore null values.

Working with null values in Boolean expressions

Expressions that combine a null value with a Boolean expression produces results that are ANSI compliant. For example:

Null values in filter conditions

If a filter condition evaluates to NULL, the function does not select the record.

Nulls with operators

Any expression that uses operators (except the string operator ||) and contains a null value always evaluates to NULL. For example, the following expression evaluates to NULL:
8 * 10 - NULL
To test for nulls, use the ISNULL function.