Technical Column

About XML Schema

XML Schema is a specification defining the structure of XML documents. It was published as a recommendation by W3C, on May 2, 2001.
"Schema" means the construction of data contained in database and the like. In XML also, the document structures such as relationships of nested elements are defined as "schema", and the structures are expressed in "schema language".
The necessity of schema can be easily understood if you think of an electronic data exchange between companies, for example. One company may show the date of delivery with <日付> tag, while another company may use ＜Date＞ tag for it. Besides, the date is expressed in the format of "2001/05/02" in Japan, while it may be expressed in the format of "02/05/2001" in other countries. The "schema" approaches these kinds of problems by standardizing document structure.
In XML 1.0 recommended in 1998, DTD (Document Type Definition) was also specified as a simple schema language. XML Schema recommended in 2001 has the following features that DTD does not:

1. ・Introduction of data types
2. While DTD only handled strings, data types such as numeric values and dates were added to XML Schema. It made electronizing of statements between companies easy.

1. ・Support more advanced structure
2. Describing in advanced structures became possible, in cases where the same tag name can be used differently in different contexts, for example.

1. ・Support namespaces
2. In DTD, using namespaces was virtually impossible. Supporting namespaces made it easier to utilize various kinds of XML documents at once.

1. ・Description of XML
2. XML Schema can be described by the same way as XML documents, while DTD format did not use tags.