Generalizations/Specializations

Generalizations/Specializations express a hierarchical relationship between entity sets, which is a concept not directly supported by the relational model (see the Relational Model chapter for the definition of the model).

Generalizations/Specializations must therefore be restructured before starting the conversion of the E-R diagram into a relational database schema.

There are 3 ways to resolve generalizations/specializations:

replace “ISA” constructs with identifying relationship sets
collapse child entity sets into the father entity set
collapse the father entity set into the child entity sets (with total generalizations only)

Notice that the following explanations treat one-level “ISA” constructs only. In case of multiple level hierarchies, one only needs to apply the chosen method to each level, recursively.

Notice also that a different method may be applied to a different level within the same hierarchy.