ITEC 3220M Using and Designing Database Systems

1. ITEC 3220MUsing and Designing Database Systems Instructor: Prof. Z. Yang Course Website: http://people.yorku.ca/~zyang/itec3220m.htm Office: Tel 3049

2. Supertypes and Subtypes • Generalization hierarchy: depicts relationships between higher-level supertype and lower-level subtype entities • Supertype: contains the shared attributes • Subtype: contains the unique attributes • Inheritance: • Subtype entities inherit values of all attributes of the supertype • An instance of a subtype is also an instance of the supertype

3. Attributes shared by all entities Attributes unique to subtype1 Attributes unique to subtype2 Supertypes and Subtypes (Cont’d) Supertype/ subtype relationships General entity type SUPERTYPE And so forth SUBTYPE1 SUBTYPE2 Specialized version of supertype

4. Supertypes and Subtypes (Cont’d) • Disjoint relationships • Unique subtypes • Non-overlapping • Indicated with a ‘G’ • Overlapping subtypes • An instance of the supertype could be more than one of the subtypes • Indicated with a ‘Gs’

5. Generalization Hierarchy with Overlapping Subtypes

6. Chapter 5 Logical Database Design and Normalization of Database Tables

7. In this chapter, you will learn: • How to transform ERD into relations • What normalization is and what role it plays in database design • About the normal forms 1NF, 2NF, 3NF, BCNF, and 4NF • How normal forms can be transformed from lower normal forms to higher normal forms • Normalization and E-R modeling are used concurrently to produce a good database design • Some situations require denormalization to generate information efficiently

8. Transforming ERD into Relations • Step one: Map regular entities • Each regular entity type in an ER diagram is transformed into a relation • The name given to the relation is generally the same as the entity type • Each simple attribute of the entity type becomes an attribute of the relation and the identifier of entity becomes the primary key of the corresponding relation

9. Example STUDENT Student_ID Student_Name Other_Attributes

10. Transforming ERD into Relations (Cont’d) • Step two: Map weak entities • Create a new relation and include all of the simple attributes as the attributes of this relation. Then include the primary key of the identifying relation as a foreign key attribute in this new relation. The primary key of the new relation is the combination of this primary key of the identifying relation and the partial identifier of the weak entity type.

11. Date_of_birth Employee_ID Employee_Name Dependent_Name Gender Has EMPLOYEE DEPENDENT Example EMPLOYEE DEPENDENT

12. Transforming ERD into Relations (Cont’d) • Step three: Map binary relationship • Map Binary one-to-many relations • First create a relation for each of the two entity types participating in the relationship, using the procedure described in step one. • Next, include the primary key attribute of the entity on the one-side of the relationship as a foreign key in the relation that is on the many-side of the relationship.

13. Order_Date Order_ID Customer_Name Customer_ID 1 Customer Submits Order (0,N) (1,1) Example M Customer Order

14. Transforming ERD into Relations (Cont’d) • Step three: Map binary relationship (Cont’d) • Map binary one-to-one relationships • First, two relationships are created one for each of the participating entity types. • Second, the primary key of one of the relations is included as a foreign key in the other relation.

15. Location Nurse_Name Nurse_ID In_charge Example Centre_Name 1 1 Care Centre Nurse (0,1) (1,1) Nurse Care Centre

16. Transforming ERD into Relations (Cont’d) • Step Four: Map composite Entities • First step • Create three relations: one for each of the two participating entities, and the third for the composite entity. We refer to the relation formed from the composite entity as the composite relation • Second step • Identifier not assigned: The default primary key for the composite relation consists of the two primary key attributes from the other two relations. • Identifier assigned: The primary key for the composite relation is the identifier. The primary keys for the two participating entity types are included as foreign keys in the composite relation.

17. Order Product Price Product_ID Order_Date Order_ID Quantity Description Order Line Example 1 (1,N) (1,1) M M (1,1) 1 (0,N)

18. Example Order Order Line Product

19. 1 M 1 Customer Vendor_ID Address Customer_ID Date Shipment_No Amount Name Shipment Vendor Example M Customer Shipment Vendor

20. Transforming ERD into Relations (Cont’d) • Step Five: Map unary relationship • Map unary one-to-may relationship • The entity type in the unary relationship is mapped to a relation using the procedure described in Step one. Then a foreign key attribute is added within the same relation that references the primary key values. A recursive foreign key is a foreign key in a relation that references the primary key values of that same relation.

21. Employee_ID Birthdate Name (1,1) M Employee 1 (0,N) Manages Example Employee

22. Transforming ERD into Relations (Cont’d) • Step six: Map ternary relationship • Convert a ternary relationship to a composite entity • To map a composite entity that links three regular entities, we create a new composite relation. The default primary key of their relation consists of the three primary key attributes for the participating entities. Any attributes of the composite entity become attributes of the new relation

23. Date Time Treatment_ Code Description Patient_Name Patient_ID Physician_Name Physician_ID Results Treatment Patient Physician Patient Treatment Example (0,N) 1 (0,N) 1 (1,1) (1,1) M M M (1,1) 1 (0,N)

24. Patient Physician Patient Treatment Treatment Example

25. Transforming ERD into Relations (Cont’d) • Step seven: Map supertype/subtype relationships • Create a separate relation for the supertype and for each of its subtype • Assign to the relation created for the supertype the attributes that are common to all members of the supertype, including the primary key • Assign to the relation for each subtype the primary key of the supertype, and only those attributes that are unique to that subtype • Assign one attribute of the supertype to function as the subtype discriminator

26. Address Date_Hired Employee_Name Employee_Number Employee_Type Stock_Option Annual_Salary Employee Salaried Employee Hourly Employee Gs Hourly_Rate Example

27. Employee Hourly_Employee Salaried_Employee Example

28. Database Tables and Normalization • Table is the basic building block in database design • Normalization is the process for assigning attributes to entities • Reduces data redundancies • Helps eliminate data anomalies • Produces controlled redundancies to link tables • Normalization stages • 1NF - First normal form • 2NF - Second normal form • 3NF - Third normal form • 4NF - Fourth normal form

29. Need for Normalization

30. Anomalies In the Table • PRO_NUM intended to be primary key • Table displays data anomalies • Update • Modifying JOB_CLASS • Insertion • New employee must be assigned project • Deletion • If employee deleted, other vital data lost

31. Conversion to First Normal Form • Repeating group • Derives its name from the fact that a group of multiple entries of same type can exist for any single key attribute occurrence • Relational table must not contain repeating groups • Normalizing table structure will reduce data redundancies • Normalization is three-step procedure

32. Conversion to First Normal Form (continued) • Step 1: Eliminate the Repeating Groups • Present data in tabular format, where each cell has single value and there are no repeating groups • Eliminate repeating groups, eliminate nulls by making sure that each repeating group attribute contains an appropriate data value

33. Conversion to First Normal Form (continued)

34. Conversion to First Normal Form (continued) • Step 2: Identify the Primary Key • Primary key must uniquely identify attribute value • New key must be composed

35. Conversion to First Normal Form (continued) • Step 3: Identify all dependencies • Dependencies can be depicted with help of a diagram • Dependency diagram: • Depicts all dependencies found within given table structure • Helpful in getting bird’s-eye view of all relationships among table’s attributes • Makes it less likely that will overlook an important dependency

36. Conversion to First Normal Form (continued)

37. Conversion to First Normal Form (continued) • First normal form describes tabular format in which: • All key attributes are defined • There are no repeating groups in the table • All attributes are dependent on primary key • All relational tables satisfy 1NF requirements • Some tables contain partial dependencies • Dependencies based on only part of the primary key • Sometimes used for performance reasons, but should be used with caution • Still subject to dataredundancies

38. Conversion to Second Normal Form • Relational database design can be improved by converting the database into second normal form (2NF) • Two steps

39. Conversion to Second Normal Form (continued) • Step 1: Write Each Key Component on a Separate Line • Write each key component on separate line, then write original (composite) key on last line • Each component will become key in new table

40. Conversion to Second Normal Form (continued) • Step 2: Assign Corresponding Dependent Attributes • Determine those attributes that are dependent on other attributes • At this point, most anomalies have been eliminated

41. Conversion to Second Normal Form (continued)

42. Conversion to Second Normal Form (continued) • Table is in second normal form (2NF) when: • It is in 1NF and • It includes no partial dependencies: • No attribute is dependent on only portion of primary key

43. Conversion to Third Normal Form • Data anomalies created are easily eliminated by completing three steps • Step 1: Identify Each New Determinant • For every transitive dependency, write its determinant as PK for new table • Determinant • Any attribute whose value determines other values within a row

44. Conversion to Third Normal Form (continued) • Step 2: Identify the Dependent Attributes • Identify attributes dependent on each determinant identified in Step 1 and identify dependency • Name table to reflect its contents and function

45. Conversion to Third Normal Form (continued) • Step 3: Remove the Dependent Attributes from Transitive Dependencies • Eliminate all dependent attributes in transitive relationship(s) from each of the tables that have such a transitive relationship • Draw new dependency diagram to show all tables defined in Steps 1–3 • Check new tables as well as tables modified in Step 3 to make sure that each table has determinant and that no table contains inappropriate dependencies

46. Conversion to Third Normal Form (continued)

47. Conversion to Third Normal Form (continued) • A table is in third normal form (3NF) when both of the following are true: • It is in 2NF • It contains no transitive dependencies

48. The Boyce-Codd Normal Form (BCNF) • Every determinant in table is a candidate key • Has same characteristics as primary key, but for some reason, not chosen to be primary key • When table contains only one candidate key, the 3NF and the BCNF are equivalent • BCNF can be violated only when table contains more than one candidate key

49. The Boyce-Codd Normal Form (BCNF) (continued) • Most designers consider the BCNF as special case of 3NF • Table is in 3NF when it is in 2NF and there are no transitive dependencies • Table can be in 3NF and fails to meet BCNF • No partial dependencies, nor does it contain transitive dependencies • A nonkey attribute is the determinant of a key attribute

50. The Boyce-Codd Normal Form (BCNF) (continued)