HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts Database Design Concepts IMAT1408 Lecture 5 Handling Many to Many relationships

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 2 Handling Many:Many Relationships Aims: –To demonstrate how to decompose many:many (M:M) relationships –To explain why M:M relationships need to be decomposed.

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 3 M:M Relationships A M:M relationship between 2 entity types must be decomposed into two 1:M relationships.

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 4 StudentModule chooses MM Becomes ModuleStudent Module Choice M M 1 1 makes is for

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 5 The Decomposition Rule A B r MM Becomes AB M M 1 1

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 6 Or - AB r MM Becomes AB M M1 1

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 7 Naming Naming the new entity type and the new relationships is sometimes not easy Often the old relationship name will help you name the new entity. In our example “chooses” became “choice” If all else fails, concatenate (join) the names of the 2 original entity types (e.g. Student Module).

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 8 Exercise Decompose this M:M relationship to form two 1:M relationships: Assign the new entity and relationship types suitable names. Doctor Patient examines M M

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 9 Solution Doctor Patient examines Patient at M 1 Appointment attends M 1

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 10 Identifiers We have seen that an entity must have an identifier The new entity type created by decomposition needs an identifier Start with a composite of the identifiers of the 2 original entity types –Need to consider carefully whether this will uniquely identify every occurrence of the new entity type.

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 11 Identifiers cont. For the second example: Doctor (doctor#,.... ) Patient (patient#,... ) So try Examination (doctor#, patient#, …..) Notice that doctor# and patient# are foreign keys (marked in bold type) as well as forming the new primary key Is this a suitable identifier?.

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 12 Identifiers cont. To decide if an identifier is suitable: Think of some other attributes for the entity Is one pair of doctor#, patient# values associated with just one value of each of these attributes?

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 13 Could a patient see the same doctor more than once? Examination (doctor#, patient#, ……………………..)

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 14 Could a patient see the doctor more than once in a day? Examination (doctor#, patient#, ………………………..)

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 15 This is getting a little complicated - maybe we should add a new key field: examination number Examination (examination#, doctor#, patient#, date, time,..) What does the underlining and bold type mean?

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 16 Why Decompose? Student(student#, name,...) Module(module#, description,...) How do we know which students are taking which modules? We don’t. StudentModule chooses MM Back to the first example Look at the original M:M relationship:

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 17 Why Decompose? cont. Decomposing gives us a new table: Student Module (student#, module#, ) Is this a suitable identifier ? Now we can list which student has chosen which module.

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 18 Exercise Actor (actor#, name,...) Film (film#, title,...) Decompose this M:M relationship Assign the new entity type an appropriate name and think of some additional attributes for it Assign the new entity type a suitable identifier. Actor Film appears in M M

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 19 Solution Actor Film Member of M 1 Cast List Actors in M 1

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 20 Summary We have looked at decomposition of M:M relationships Discussed how to identify a unique identifier.

HNDComputing – DeMontfort University  DeMontfort University 2011 Handling Many to Many relationships wk5 Database Design ConceptsDatabase Design Concepts 21 Reference Data Analysis for Database Design by D R Howe