The nested relational model is an extension of the relational
model in which domains may be either atomic or relation-valued. This allows a
complex object to be represented by a single tuple of a nested relation -
one-to-one correspondence between data items and objects.

A drawback of relational model is that it can represent data
only in the simple first normal form. There are two proposals to improve it.

1. Since the concept of dependencies has been
proposed to eliminate information loss accompanying updating, databases without
updating such as those used only for searching do not have to be in the first
normal form.

2. Decompositions
of relations according to multi-valued dependencies are not always necessary in
order to prevent errors in updating.

There are two advantages. I.e. the expressions of data in the model are natural or efficient and less join operations are necessary, which means data processing becomes generally faster.

**Nested Relation Type (NRT) model, allows, in addition to
system- defined null types, user-defined null types.**

The NRT model extends the nested relational model by
incorporating a form of built-in inheritance. This allows defining a partial
order between nester-relations types and a partial order between the data
values of these types. By utilizing these partial orders, one defines an
instance, over a NRT, to be incomplete when its information content may
increase.

In addition, algebra is defined for the NRT model, called the
NRT algebra. Which is shown to supersede known as algebras for relations with
nulls and for nested relations by showing faithfulness to these algebras.

In relational databases, all relations are at least in
First-Normal-Form (INF) which requires all attributes to have atomic That is,
elements of the domains are considered to be indivisible units.

In the nested relational model as an extension of the
relational model domains may be either atomic or relation valued. That is, an
attribute value of a tuple can be a relation.