Database Modeling and Design

106 CHAPTER 5 Transforming the Conceptual Data Model to SQL
5.3 Summary
Entities, attributes, and relationships in the ER model and classes,
attributes, and associations in UML can be transformed directly into SQL
(SQL-99) table definitions with some simple rules. Entities are transformed
into tables, with all attributes mapped one-to-one to table
attributes. Tables representing entities that are the child (“many” side) of
a parent-child (one-to-many or one-to-one) relationship must also
include, as a foreign key, the primary key of the parent entity. A manyto-many
relationship is transformed into a table that contains the primary
keys of the associated entities as its composite primary key; the
components of that key are also designated as foreign keys in SQL. A ternary
or higher-level n-ary relationship is transformed into a table that
contains the primary keys of the associated entities; these keys are designated
as foreign keys in SQL. A subset of those keys can be designated as
the primary key, depending on the functional dependencies associated
with the relationship. Rules for generalization require the inheritance of
the primary key from the supertype to the subtype entities when transformed
into SQL tables. Optionality constraints in the ER or UML diagrams
translate into nulls allowed in the relational model when applied
to the “one” side of a relationship. In SQL, the lack of an optionality
constraint determines the not null designation in the create table definition.
5.4 Literature Summary
Definition of the basic transformations from the ER model to tables is
covered in McGee [1974], Wong and Katz [1979], Sakai [1983], Martin
[1983], Hawryszkiewyck [1984], Jajodia and Ng [1984], and for UML in
Muller [1999].
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