Database Modeling and Design

5.2 Transformation Steps 103
there are no common attributes and no integrity constraints to maintain.
The main function of aggregation is to provide an abstraction to
aid the view integration process. In UML, aggregation is a composition
relationship, not a type relationship, which corresponds to a weak entity
[Muller, 1999].
5.1.5 Multiple Relationships
Multiple relationships among n entities are always considered to be completely
independent. One-to-one, one-to-many binary, or binary recursive
relationships resulting in tables that are either equivalent or differ
only in the addition of a foreign key can simply be merged into a single
table containing all the foreign keys. Many-to-many or ternary relationships
that result in SQL tables tend to be unique and cannot be merged.
5.1.6 Weak Entities
Weak entities differ from entities only in their need for keys from other
entities to establish their uniqueness. Otherwise, they have the same
transformation properties as entities, and no special rules are needed.
When a weak entity is already derived from two or more entities in the
ER diagram, it can be directly transformed into a table without further
change.
5.2 Transformation Steps
The following list summarizes the basic transformation steps from an ER
diagram to SQL tables:
• Transform each entity into a table containing the key and nonkey
attributes of the entity
• Transform every many-to-many binary or binary recursive relationship
into a table with the keys of the entities and the
attributes of the relationship
• Transform every ternary or higher-level n-ary relationship into a
table
Now let us study each step in turn.
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