The concept of functional dependencies plays an important role in relational databases where it is used in integrity enforcement and database design. The corresponding concept, functional dependencies in XML (XFDs), has been widely investigated over the past few years. Although some proposals have been made and most of them are based on path that is a crucial factor for the expressive power of XFDs, there seems to be no consensus on how to specify them. In this paper, we survey the previous work in terms of the approaches they employ. Some related issues are discussed. In particular, we analyze the effects of null values and how to compare element nodes with complex type. We also discuss the logical implication and normalization for XML in brief. A set of requirements for a good XFDs definition are concluded.
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