NULL Predicate
The NULL predicate indicates whether the document has a value for the indicated column. The NULL predicate syntax follows:
...WHERE <column> IS [NOT] NULL
The optional NOT keyword negates the result. The column can be a regular or delimited identifier.
Important To test whether a column has the NULL value, you must use the NULL predicate. It is not legal to use the NULL value in a comparison predicate. "WHERE column IS NULL" is the proper method. "WHERE column = NULL" is not allowed.
Example
The following example returns documents that have no related documents.
...WHERE System.Video.Director IS NULL