However, in XML it cannot be omitted. The order of the fields is again immaterial. If d is omitted, it means the value of the fields is the empty string. Null values are represented by the absence of the field. Note that this is different from leaving the field empty, which indicates that the field contains a string of length zero.
Null values have special properties in relational databases.
Strong typing Tim Bray has written a proposal for adding strong typing to XML, using a set of fixed attributes. This will allow an application that knows about these attributes to check the content of each field. The relational data model is explained in many books. Principles of database and knowledge-base systems, volume I. Computer Science Press, Rockville, Maryland, 1988. An introduction to database systems, volume I. Addison-Wesley, Reading, MA, 1986.