12.3. String Datatypes
The string datatype is the archetype for all string-oriented datatypes. It is effectively completely defined by these facts:
The mapping from the lexical space to the value space is the identity function.
Both spaces consist of all possible strings of characters of finite length.
As a result, all derived types are completely defined by restrictions on the space. Because the mapping between them is the identity, any restriction on either space automatically induces the same restriction on the other.
A few datatypes nominally derived from string have aspects that do not follow from their derivation. We discuss them separately in Section 12.3.3.
12.3.1. The Built-in string Datatype
When a schema component specifies a string ...