1.Flat Structure : A Flat stucture is a structure that refers to an elementary type or reference type. In other words, flat structures only refers to the data-element or predefined data-type. They don't refer to any other structure or Table-Types. In the database table , only flat structure can be included as sub-structures.
2.Nested Structure : A nested structure is a structure that refers to at least one structure. Thus nested structure can refer to the data element and predefined data type, and to another structure (which is mandatory). they don't refer to the table type.
3.Deep Structure : A Deep structure is a structure that refers to at least one table type along with data-elements, predefined data-types and structures.