The theoretical basis of the relational database is a collection, and the basic definition of the collection is a group of elements that are not repeated. The XML database is still lacking the corresponding theory to eliminate data redundancy. Another important reason why the relational database can succeed is that it uses a flat table form, and a large amount of use in the application is a flat table, so the database table is in the optimal expression of data, using XML expression will only increase unnecessary Complexity. The basic hypothesis of the planar table is the same as the structure of all entries (with a header).
The XML represents the form of a form that does not exist such a hypothesis, so many times, it is not possible to do effectively according to the Shape of the data. Of course, XML Schema and other techniques are rapidly developing. After the corresponding metadata description and use techniques are gradually mature, the XML processing method will be improved.
XML technology is the representative of the current meta-language. Its most important technical advantage is that it is a language that people can easily understand. It is a channel shared by human machine! It is not suitable for expressing complex associations in the application. In particular, most of the APIs of most manipulating XML are documentation, and the data types are all string, which has caused difficulties in program applications.
