Datatypes

Until now, we have copied the DTD, with some simplification. But, one can:

• define constraints on the data types (eg, an attribute shall be a positive integer)
• define hierarchies of data types (eg, a type is a subtype of another one by restriction)

The ability of defining complex data is one of the main strengths of schemas!

For example: phone numbers should include only numbers, first names only characters, zip codes follow national patterns,...