Expression must return Boolean values. If it evaluates to True on a row, the row is included in the data view. If it evaluates to false, the row is excluded. For example, expression "City = 'London'" selects all rows where field City equals 'London'.

To select rows with null values in a certain field, use the following expression:

"IsNull(Field1, 'Null Field') = 'Null Field"

Reference

C1DataView Class
C1DataView Members
C1DataObjects Expressions