Introduction: An important feature of object-relational and object-oriented databases is their capability of storing, accessing, and modeling complex structure-valued data, such as set- and list-valued data and data with nested structures.
A set-valued attribute may be of homogeneous or heterogeneous type. Typically, set-valued data can be generalized by (1) generalization of each value in the set to its corresponding higher-level concept, or (2) derivation of the general behavior of the set, such as the number of elements in the set, the types or value ranges in the set, the weighted average for numerical data, or the major clusters formed by the set. Moreover, generalization can be performed by applying different generalization operators to explore alternative generalization paths. In this case, the result of generalization is a heterogeneous set.
Example: Generalization of a set-valued attribute. Suppose that the hobby of a person is a set-valued attribute containing the set of values {tennis, hockey, soccer, violin, SimCity}. This set can be generalized to a set of high-level concepts, such as {sports, music, computer games} or into the number 5 (i.e., the number of hobbies in the set). Moreover, a count can be associated with a generalized value to indicate how many elements are generalized to that value, as in {sports(3),music(1), computer games(1)}, where sports(3) indicates three kinds of sports, and so on.
A set-valued attribute may be generalized to a set-valued or a single-valued attribute; a single-valued attribute may be generalized to a set-valued attribute if the values form a lattice or “hierarchy” or if the generalization follows different paths. Further generalizations on such a generalized set-valued attribute should follow the generalization path of each value in the set.
List-valued attributes and sequence-valued attributes can be generalized in a manner similar to that for set-valued attributes except that the order of the elements in the list or sequence should be preserved in the generalization. Each value in the list can be generalized into its corresponding higher-level concept. Alternatively, a list can be generalized according to its general behavior, such as the length of the list, the type of list elements, the value range, the weighted average value for numerical data, or by dropping unimportant elements in the list. A list may be generalized into a list, a set, or a single value.
