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Fundamentals/ICY: Databases 2012/13 WEEK 5

Fundamentals/ICY: Databases 2012/13 WEEK 5. John Barnden Professor of Artificial Intelligence School of Computer Science University of Birmingham, UK. Reminder of Week 4. 1:1 Connectivity between Tables. People. 1:1: that is, no more than one phone allowed per person, and vice versa.

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Fundamentals/ICY: Databases 2012/13 WEEK 5

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  1. Fundamentals/ICY: Databases2012/13WEEK 5 John Barnden Professor of Artificial Intelligence School of Computer Science University of Birmingham, UK

  2. Reminder of Week 4

  3. 1:1 Connectivity between Tables People 1:1: that is, no more than one phone allowed per person, and vice versa. Phones Note: the representation is still asymmetric in that the People table mentions phones but not vice versa – symmetry would create extra redundancy. NB: Biggles has no phone listed, and 0121-440-5677 has no person recorded. Suggests a possible reason for not combining such tables.

  4. 1:M Connectivity between Tables More than one employee allowed per organization, but no more than one employer per person. NOTE direction of use of the foreign key. Why so?? People Organizations

  5. New for Week 5

  6. M:N Connectivity between Tables • IF we represent M:N connectivity in a similar way to 1:M, then we can expect that • in the People table: some people will have each several employers listed • or in the Organizations table: an organization will have several employees listed • or both. • This is a problem. Why?

  7. M:N Connectivity between Tables, contd. • Because of this problem, an M:N relationship is usually broken up into two 1:M relationships. • This means introducing an extra “bridging”or “linking” or “composite” entity type (hence table) to stand between the two original ones.

  8. M:N -- a person may be employed by more than one organization and an organization may employ more than one person PEOPLE EMPLOYMENTS each = person-id + organzn-id possibly plus other attributes does not have an EMPL ID attrib. ORGANIZATIONS does not have a PERS ID attrib

  9. M:N Connectivity between Tablesusing a Bridging Entity Type People Employments Organizations

  10. Strong (or Identifying) Relationships • A relationship from entity type A to entity type B, mediated by having A’s primary key (PK) as a foreign key in B, is strongwhen B’s PK contains A’s PK. (incl. the case of B’s PK just being A’s PK). • So, B entities are defined in terms ofA entities. • E.g., A = Customers, B = Dependents, where • A’s PK is: CUST_ID • B’s PK is: CUST_ID, FIRST_NAME, CONNECTION. • So a PK value in B could be (1698674, Mary, child) , meaning that this entity is the child called Mary of person 1698674 in the Customer table.

  11. a Strong Relationship Customers (the “A” type) Strong relationship going from Ato B (we could say: “B is strongly dependent on A”) Dependents (the “B” type)

  12. Weak (or Non-Identifying) Relationships • A relationship is weakwhen it isn’t strong! • So, most relationships are weak. • Note that strength/weakness is directional: the People to Dependents relationship (above) is strong, but the Dependents to People relationship is weak. • Exercise: Can a relationship be weak in both directions? • Exercise: Can a relationship be strong in both directions?

  13. Weak Entity Types • A weak entity typeE is one such that there is a relationship Rfrom some other entity typeFtoE that satisfies the following two conditions: • R is strong. • E is existence-dependent on F via R. • That is, an Eentitye can only exist in the database if some F entity in the database bears relationship R to e. • This actually just means that R —1 (the R relationship but in the E to F direction) is mandatory.

  14. Weak Entity Types, contd. • So on a previous slide, Dependents is weak, because there is a strong relationship to it from Customers, and Dependents is existence-dependent on Customers via this relationship. • Mary’s existence in the databaseas a member of Dependentsrelies on the existence of customer 1698674 in the database. • But this doesn’t mean Mary would vanish from the world if customer 1698674 left the database or even if that customer were to vanish from the world. • And indeed Mary could herself be an entity in type Customers, and could remain so even if 1698674 left the DB or really vanished from the world.

  15. Strong Entity Types • A strong entity type is one that is not weak! …. • So, in particular, any entity that receives only weak relationships from other entity types is strong. • So the usual case is for an entity type to be strong. • And any entity type that is not existence-dependent on anything is strong.

  16. Mental Exercises for You • What about the Employments bridging type we introduced? • Although it might look at first as though a strong relationship necessarily implies existence dependence, it doesn’t. Why not? • One entity type can be existence-dependent on another without therefore being weak. Why?

  17. Entity Relationship (ER) Model(s) and Diagrams (ERMs and ERDs)

  18. The Entity Relationship Model • Introduced by Chen in 1976 • Most widely used “conceptual model” of DBs. • “The ER model” : general meaning = just the idea of thinking of things as composed of entities, attributes and relationships. • We also say that applying this approach in a particular case gives rise to an “ER model” of the specific environment of interest. • Diagrams based on the/a model are a widely accepted and adopted graphical approach to database design. • The/an ER model has nothing intrinsically to do with diagrams!!! Let alone any specific sort of diagram!!!

  19. A Conceptual Model • ER model that provides high-level, manager/user-friendly view of the database • Basis for identification and description of main data objects and relationships, avoiding details

  20. Entity Relationship Diagrams (ERDs) • An ER model of an environment forms the basis of an ER diagram (ERD) or several ERDs. • There are several markedly different styles of ERD, and for each main style there are several variants. • And the style in the module handouts will differ somewhat from that in the textbooks and these lectures • An ERD depicts (some of) the ER model’s entity types, attributes and relationships, and (depending on the diagram style) varying amounts of other info such as connectivities, cardinalities, keys, weakness, …

  21. Quick Flavour of Two Styles of Diagram

  22. The Completed Tiny College ERD

  23. Relationships: The Basic Chen ERD-let

  24. Relationships: The Basic Crow’s Foot ERD-let

  25. Caution • In previous two diagrams, each relationship was mandatory in both directions. • But saying “1:1”, “1:M” or “M:N” does not of itself imply mandatoriness in either direction. In particular, don’t be deceived by the “1” here – it’s not a minimum. • We will see in a minute how to draw optional (non-mandatory) relationships.

  26. A Model for Tiny College

  27. 1:1 Relationship Between PROFESSOR and DEPARTMENT(mandatory in both directions)

  28. 1:M Relationship Between PAINTER and PAINTING (mandatory in both directions)

  29. Tables for that 1:M Relationship

  30. M:N Relationship between STUDENT and CLASS (both ways mandatory):preliminary ERD (conceptual level)

  31. Conversion to Two 1:M Relationships (towards “logical” level)

  32. A Bridge (or Composite) Entity Type • Its table is called a linking table (or bridging table) • Its primary key is composed of the primary keys of each of the entity types to be connected • Those keys are also foreign keys in the bridge type • Linking table may contain multiple occurrences of each foreign key value • May also contain additional attributes that play no role in the bridging as such

  33. Direct Impln of M:N Relationship between STUDENT and CLASS

  34. Converting the M:N Relationship into Two 1:M Relationships

  35. Connectivity and Cardinality in an ERD

  36. Relationship Participation • Optional [in a particular direction, X to Y]: • an X entity occurrence does not requirea corresponding Y entity occurrence • i.e. the minimum number of Ys per X is 0 • Mandatory [in a particular direction, X to Y]: • an X entity occurrence requiresa corresponding Y entity occurrence • i.e. the minimum number of Ys per X is 1 or more

  37. Drawing Optionality NOTE: the dashing of the line is NOT because of the optionality, but because of weakness

  38. The Chen Representation of the Invoicing Problem

  39. The Crow’s Foot Representation of the Invoicing Problem

  40. The Attributes of the STUDENT Entity

  41. Attributes

  42. Weak Entity Types in ERDs

  43. A Weak Entity in an ERD

  44. A Weak (Non-Identifying) Relationship

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