Creates a SQL literal for the given value, suitable
for comparison to a column of this indicated type.
If the value is not suitable for the column type
(e.g., not a number for a SQLExactNumeric), NULL
A regular expression that covers the lexical form of
all values of this datatype (in their RDF representation).
This is especially important for types that are not mapped
to a typed literal but to plain/xsd:string literals.