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 muticolumn table (with a column for each attribute of the object type).
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 Varrays that are inlined to the parent table.
TopLink supports nested table through the NestedTableMapping class. They represent a collection of object references in Java, similar to a OneToManyMapping or ManyToManyMapping. However, the nested table mapping does not require foreign key information (as with a one-to-many mapping) or the relational table (as with a many-to-many mapping).
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