Super-Type & Sub-Type Entities—Topics

Super-Type & Sub-Type Entities—Topics • Problems needing subtype entities • Nature of the solution • Variations—Specialization and Completeness • Subtype Identifiers • Implementing • Special Topics • Using Super- and Sub-Types • Performance Considerations

Supertype & Subtype Entities • Some entities have records that come in various ‘flavors’. • StudentsDoctoral, Masters, Undergraduate • ProductsSerial-numbered, perishable, animals, etc. • EmployeesSalaried, hourly, managerial, part time • Pet Store Products Food, animals, accessories • These entity sets have two types of attributes • Attributes common to every occurrence • Attributes required by one or more subtypes but not used by all occurrences of the entity

Why is This a Problem? • Variations on an entity create a space problem • If we put all possible attributes for all possible variations (subtypes) in one entity we will waste unused fields in most records • Sport attribute for students who are not athletes

Supertype & Subtype Entities (cont.) • Subtypes also create relationship problems • Some relationships will only be with a subtype of the entity, not with all types • In a pet store a veterinarian will inspect the animals inventory items but probably not the turtle food

Supertype & Subtype Entities • It is common to split upentities with variationsinto a supertype andsome subtypes • Supertype containsattributes common toall occurrences • Subtypes contain attributes needed by the subtype ERD Notation Visio Equivalent

An Example • Cash is a PaymentTypebut needs no special attributes • Partial Specialization (coming up) • Payment ID is PK of all entities • Payment ID is also FK insubtype entities • In SQL Server besure to set parent this way when implementing relationships

Need for Subtypes • Subtypes are used when an identifiable subset of occurrences have a need for fields not needed by all occurrences • Many occurrences will have empty attribute values • An occurrence’s membership in the identifiable subset must be observable • It is known whether a student is registered as an athlete • But there is no obvious distinction to distinguish ‘local’ students from ‘transient’ students

First Variation on Super-/Subtypes • Completeness Constraint • Must every supertype occurrence have at least one occurrence in one of the subtypes? • Total specialization means thata subtype occurrence must exits • Indicated with a double lineto the connecting circle • Partial specialization means thata subtype need not exist • Indicated with a single line tothe connecting circle

Total Specialization Completeness Constraint • Total specialization means that every record in the supertype must have a matching record in one or more subtypes • Relatively rare (in my experience) but possible • Model in Visio using a thicker descending line (use Format Line) • (Visio doesn’t do double lines) • Increase thickness by two levels • Watch for SQL Server modeling later

Partial Specialization Completeness Constraint • Some records in supertypes may have no matching subtype records • Their subtype groups do not needspecial attributes • But membership in a groupmay still be important andtracked • It is possible for a suptertype to haveonly one subtype group

Your Turn • Model the products in a home improvement store as a supertype/subtype relationship • Identify categories and any specialized attributes needed

Second Variation on Super-/Subtypes • You must also determine whether a supertype occurrence can be found in more than one subtype • A disjoint relationship meansthat a supertype occurrence can only be found in one subtype • An overlap relationship means that a supertype occurrencecan be found in multiple subtypes(E.g., some universities have ajoint J.D./MBA program) “d” “o”

Disjoint Relationships • A registered vehicle canonly be of one type

Overlap Relationships

Subtype Identifiers • The supertype entity must indicate which (if any) subtypes are used • Disjoint subtypes can use one attribute with a code to indicate the type of subtype • Value of the attribute (‘Cash’, ‘Check’, ‘CC’) identifies the subtype • Remember that some subtype identifiers (‘Cash’ here) may have no subtype entities • Sometimes this value may be blank (not part of any group)

Subtype Identifiers (cont.) • Overlapping subtypes must use a collection of yes/no attributes, one for each possible subtype • Setting attribute to true/yes in a record indicates that a matching subtype record exists • Leaving all to false/no indicates no matching subtype (partial specialization)

Subtype Identifiers (cont.) • A subset of subtypes may be disjoint while others are overlap

Subtypes of Subtypes • It is possible to have subtypes of subtypes • Model products in a pet store where some are inanimate, some are food, some are live and of the live animals some are tracked individually… • Cute puppies with wet noses • Cats • … and others are not • Goldfish • Mice • … and some are sold as food • Cute little mice as food for slithering scaly snakes

Some Caveats • ST/ST determined at the group level. Individual records may not have values for all fields • More than one subtype may have the same field in it • Field goes in subtype entities if not every subtype group needs it • Consider eliminating subtypes if they have only one or two attributes • Roll their attributes back into the suptertype and accept wasted space • Consider if a large proportion of the population • Consider if frequently accessed

Implementing Super-/Subtypes • There is a Mandatory-1:Optional-1 relationship between entities in a super and subtype relationship • Mandatory at supertype end • Optional at each subtype end • Each subtype occurrence (record) has identifier attribute values that exactly match a record in the supertype (but not vice-versa) • All entities have the same primary key/ identifier attributes • PK in the subtype is also the FK from supertype • Special case of a weak entity

Implementing in SQL Server—Table Design

Implementing in SQL Server—Relationships PK is also FK

Implementing in SQL Server—Diagrams • Arrange in org-chart hierarchy • Gives visual cue that this is a ST/ST relationship • You will need to wrestle with the relationship lines a little • Note Key symbols at both ends of the lines • Indicates 1:1 Cardinality

Subtypes of an Unimplemented Supertype • Many, many data models will have records that could be subtypes of a supertype that is not implemented • For UCF a “Person” entity could have subtypes • Student − Donor • Faculty − Contractor • Tend to not implement this Person supertype unless the entities are regularly queried together • Occasional queries can be supported with a UNION query

Subtypes and Object Oriented Design • Super- and Sub-type design exactly corresponds to the philosophy of inheritance in object oriented design • If programming using an OO approach you will almost always implement objects with inheritance to match super- and sub-type design • You can also implement inheritance for the unimplemented supertype discussed in the previous slide, even if not implemented in the DB design

Using Super-/Sub-type Tables • Application logic and SQL for super-and sub-type tables becomes more complex • Inserts must test the subtype identifier to determine where to add records • Always to the supertype • Decide which (if any) subtype(s) • Similar for Updates

Using Super-/Sub-type Tables (cont.) • Retrieval also complex • You cannot simply join the supertype with all subtypes since no records will be returned if a subtype has no match • Why won’t the following work? SELECT Payment.*, Check_Payment.*, CC_Payment.* FROM Payment, Check_Payment, CC_Payment WHERE Payment.PaymentID = Check_Payment.PaymentID AND Payment.PaymentID = CC_Payment.PaymentID AND Payment.PaymentID = 1472

Using Super-/Sub-type Tables (cont.) • Two query approaches • Use conditional logic • Use Left/Right Outer Joins SELECT Payment.*, Check_Payment.*, CC_Payment.* FROM Payment LEFT JOIN Check_Payment ON Payment.PaymentID = Check_Payment.PaymentID LEFT JOIN CC_Payment ON Payment.PaymentID = CC_Payment.PaymentID WHERE Payment.PaymentID = 1472

Performance Considerations • Because of the performance considerations and complexity of Super- and Sub-types you will regularly consider eliminating subtypes • Roll up their attributes into the super-type and accept the wasted columns • Arguments for retaining subtypes • Several unique attributes, especially large (text) ones • Relatively few records in the subtype (compared to overall number of records) • Relatively few transactions use the subtype • Look at vertical partitioning later in the course

Super-Type & Sub-Type Entities—Topics