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# Object-Relational Nested Table Mapping

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Nested table types model an unordered set of elements. These elements may be built-in or user-defined types. You can view a nested table as a single-column table or, if the nested table is an object type, as a multicolumn table (with a column for each attribute of the object type).

TopLink maps nested tables with the `NestedTableMapping` class. It represents a collection of object references in Java. Because of the object-relational support of the database, nested table mapping does not require foreign key information (as with a one-to-many mapping) or a relational table (as with a many-to-many mapping).

Typically, nested tables represent a one-to-many or many-to-many relationship of references to another independent structure. They support querying and joining better than the `VARRAY` types that are in-lined to the parent table. TopLink supports mapping a nested table of `REF` types only. TopLink does not support nested tables of basic or other structured data types–use array (see "Object-Relational Array Mapping") or object array (see "Object-Relational Object Array Mapping") mappings instead.

See "Configuring an Object-Relational Nested Table Mapping" for more information.