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Subtypes and supertypes can participate in relationships (Notice in figure how For example, we might have an entity Contract, subtyped into Renewable . DDS4L1 Supertypes and Subtypes Supertype Example • The same applies to relationships. • Subtypes inherit all attributes and relationships of. Primary keys are unique identifiers of the relation in question. Examples include employee numbers, social security numbers, etc. This is how we can guarantee.
But in Cthis is disallowed, although we can simulate the paradigm with interface inheritance and by following well-established patterns. In databases, diamond pattern multiple inheritance is also problematic, in different ways; I defer discussion to the design phases in the coming articles. For now, keep in mind that it is always best to find a workaround for multiple inheritance whenever possible.
Here are some strategies that are used in different scenarios: Make multiply derived entities singly derived by showing only one inheritance edge connecting it directly to the supertype. Split the supertype into two or more in a one-one relationship. Refactor the design into different entity classes that don't result in multiple inheritance. Refactor the entities into a role playing model. Represent multiply derived entities as association classes intersection entities.
The first is never valid--I put it in to make sure you're never tempted to do it.
The sharing of attributes and relationships that the entity class does with other subtype entities is no longer clear, and this can result in data integrity issues in the physical design. Later in the series I'll expand the discussion. I won't elaborate on the next three other than to say that they are not applicable to our building example.
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- Database Management Systems
- Basic notation for supertype/subtype relationships
Here is a graphic depicting the Storefront Apartment as an association class; the rest of the diagram is unchanged: The store and apartment are in a one-one relationship, the diamond represents the n-ary association between the three, and the association class extends the association with specialized attributes and relationships.
But because the meaning is really the same as diamond-pattern multiple inheritance, we've only obscured the fact; to my mind something is lost when we show a subtype as not being a full part of the supertype hierarchy. In the next article, we'll see that the CASE tool functionality that transforms the conceptual to logical design will have problems with this style as well; it will correctly write in the shared key for all entity classes, but will not make it the primary key for the association class.
Our example shows that there are true cases of multiple inheritance, and trying to design around them may only gloss over issues that won't be well-handled in the physical design. Our inheritance tree has devolved into a directed graph of one root.
Completeness and Exclusivity While we chose graphs for our discussion on inheritance, the next two properties are better described using sets. To understand them, we switch focus, terminology, and diagrammatic convention. If we think of each box as being a set, then we are seeing a proper subset relationship but without the possibility of overlap. One implication of this is that as we generalize, the population goes up, and down as we specialize; in fact each member of an inner subtype box must be present in all containing boxes including the supertype.
Because boxes cannot overlap, the tool seemingly precludes multiple inheritance, but when we get to conceptual modeling in the second article, I'll show you how we'll put it in. The sets, however, do not contain sets of attributes and relationships; if they did, then the outermost set would correspond to a tree leaf, the innermost, the root, the intermediate, those on the ancestor path from leaf to root, and we would therefore need one diagram for each leaf node.
Not a good representation.
Tables for Supertype and Subtype Entities in Oracle Communications Data Model
No; our focus now is on entity membership. In the conceptual model, the main objects of interest are the business entities themselves. For this discussion, let's refer to them as entity sets, as they are sometimes known, and each of their members as the entities. In our running example, supertype Building would be the outermost entity set, Commercial and Residential, being proper subsets or subtypeswould be rounded rectangles nested adjacently inside, and so on.
The supertype must always be the outermost box. In the topics next, I've written in sets in place of entity set names. Completeness For these discussions, an innermost entity set is one that doesn't properly contain another; alternatively, a subtype from which no other subtype is derived.
This is the condition for completeness: In other words, every entity in the supertype entity set must be found in an innermost entity set. In the diagram, the first set of sets is complete or exhaustive because all four entities in the superset are also in the innermost sets.
Modeling Supertypes and Subtypes: Part 1 - CodeProject
The second two, though, are incomplete: Besides stopping short of communicating the full design, incompleteness has implications for the physical design itself. In that modeling stage we would like full control over which entity sets become tables and which are merged into others. But any entity set--supertype or subtype--whose descendants cannot represent all of its entities must be realized as a table, or one of its containing sets must be.
We'll see this limitation when we examine the physical design in Part III. The moral of the story: Exclusivity This is the condition for exclusivity. In the equation the set builder is looking for entities that are in two different subtype entity sets that are not in the proper containment relationship, and if the resulting set is empty, then all subtypes are exclusive--i.
We rule out proper containment so as not to consider entity sets in the same specialization path--if we didn't the condition would always be met. For our building hierarchy example, the building at 1 Main Street is a storefront apartment.
By the formula, we can't compare memberships in pairings of the Commercial, Retail Store, and Storefront Apartment entity sets because of the containment restriction, and the same applies on the residential side.
But we will find the building at 1 Main in the pairing of the commercial entity set and either the residential or apartment sets, and in other valid combinations, and so the generalization hierarchy is not disjoint. I use forms of the terms mutually exclusive, overlapping, and disjoint to describe the same concept. It is important to note that Storefront Apartment is ruled out of the test for overlap.
In this analysis, entity set containment is really another way of expressing specialization paths leading from the root supertype. In your example, do you expect a Bike or a Plane etc The Information Engineering notation differentiates between inclusive and exclusive subtype relationship.
IDEF1X notation, on the other hand, doesn't directly recognize this difference, but it does differentiate between complete and incomplete subtype which IE doesn't. Physical Representation Unfortunately, practical databases don't directly support inheritance, so you'll need to transform this diagram to real tables. There are generally 3 approaches for doing so: Put all classes in the same table and leave child fields NULL-able.
No JOINing, so some queries can benefit. Can enforce parent-level keys e. Can easily enforce inclusive vs. There have been literally dozens of proposed enhancements.
Modeling Supertypes and Subtypes: Part 1
We cover the most significant ones in these notes. This is the concept of categorizing or generalizing between supertypes and subtypes of entities. As an example, a CAR entity can have several subtypes, e. The Supertype entity is connected to the Subtype entities through lines and a circle. The U-symbol indicates the Subtype is a subset of the Supertype. All of attributes of the Supertype are also common to the Subtype. Each Subtype entity has attributes that are unique to that Subtype - that is what makes the entity a subtype.
All entities share a common identifier attribute. Subtypes inherit all of the attributes of the Supertype.