Transformation to Simplified ER Diagrams

For simplicity of exposition, especially in the translation of ER diagrams to MCT schemas in the paper, it is useful to focus attention on simplified ER diagrams, that contain only entity types, binary relationship types between distinct entity or relationship types, and atomic attributes. Arbitrary ER diagrams can be translated into such simplified ER diagrams by repeatedly applying the following transformations.

• A k-ary (k>2) relationship type R between k entity (or relationship) types E₁,..., E_k is replaced by a new entity type E_R, with k new relationship types R₁, ,... R_k, where each R_i is a 1:n binary relationship type between E_i and E_R.
• A binary relationship type R in which an entity (or relationship) type E participates more than once is replaced by a new entity type E_R, with two new binary relationship types R₁, R₂, where each R_i corresponds to one of the original roles of R, linking E and E_R. The cardinality of an R_i is the same as that of R's original role.
• Composite attributes are replaced by new entity types that are linked to their original entity type by new binary relationship types. This is done since the XML data model does not support composite attributes to be associated with elements.

Note that the TPC-W ER diagram of Figure 1 in the paper is already in the desired simplified form.