Normalisation. Connolly, Ch. 13

Transcription

1 Normalisation Connolly, Ch. 13 1

2 Overview Normalisation Steps 1NF Removing repeating groups 2NF Partial dependencies Removing partial dependencies 3NF Transitive dependencies Removing transitive dependencies 2 2

3 Normalisation A formal technique for analysing relations based on their primary key and functional dependencies. [Codd]. Process Analyse each relation using a set of rules If a relation does meet a rule Decompose the relation into two or more relations 3 3

4 Normalisation Steps Each step in the process reduces the update anomalies that can occur. 0NF 1NF 2NF 3NF BCNF 4NF 5NF 4 4

5 Unnormalised Form (0NF) A 0NF relation may have: Multivalued attributes Some attributes have more than one value Composite attributes Some attributes consist of multiple parts Nested relations Some attributes contain another relation 5 5

6 0NF Example Proj-id, Proj-Start-Date, Location and Weeks-on-Project contain multiple values 6 6

7 First Normal Form (1NF) Definition A relation in which the intersection of each row and column contains one and only one value. No repeating groups A 1NF relation does not contain Multivalued attributes Composite attributes Nested relations 7 7

8 Converting 0NF to 1NF Remove repeating groups Start with a 0NF relation which contains... a key that identifies a set of attributes and a set of attributes that repeat Remove the repeating attributes and place them in a separate relation Include a copy of the original key in the new relation 8 8

9 1NF - Identify the Repeating Group Key Repeating Group 9 9

10 1NF - Remove the Repeating Group Emp1 EmpProj

11 1NF Relations Emp1(emp-no, emp-name, dept, manager) EmpProj1(emp-no, proj-id, proj-start-date, location, weeks-on-project) 11 11

12 Second Normal Form (2NF) Definition A relation that is in 1NF and every non-primarykey attribute is fully dependent on the primary key. Remove partial dependencies A 2NF relation is one that Is in 1NF and contains no partial dependencies 12 12

13 Partial Dependency on the Key An attribute that is dependent on part of the key (A, B, C, D) AB C A D D is partially dependent on the key (AB) 13 13

14 Converting 1NF to 2NF Identify all partial dependencies Ignore all relations with a single attribute primary key Emp1 This relation is in 2NF Create a new relation Remove the partially determined attribute Copy part of the primary key 14 14

15 2NF - Identify Partial Dependencies EmpProj1 Proj-Start-Date is dependent on part of the key

16 2NF - Remove Partial Dependency EmpProj2 Remove the proj-start-date attribute. Proj2 Create a relation with the proj-start-date attribute and a copy of the proj-id attribute

17 2NF Relations Emp2(emp-no, emp-name, dept, manager) EmpProj2(emp-no, proj-id, location, weeks-on-project) Proj2(proj-id, proj-start-date) 17 17

18 Third Normal Form (3NF) Definition A relation that is in 2NF and every non-primarykey attribute is dependent on the primary key only. Remove transitive dependencies. A 3NF relation is one that Is in 3NF and contains no transitive dependencies 18 18

19 Transitive Dependency on the Key An attribute that is dependent on an attribute other than the key (A, B, C, D) AB C C D D is transitively dependent on the key (AB) through C 19 19

20 Converting 2NF to 3NF Identify all transitive dependencies Ignore all relations with two attributes Proj2 For each transitive dependency Create a new relation Remove the transitively determined attribute Copy the determining attribute 20 20

21 3NF - Identify Transitive Dependencies Emp2 Manager is dependent on a non-key attribute

22 3NF - Remove Transitive Dependency Emp3 Remove the manager attribute. Dept3 Create a relation with the manager attribute and a copy of the dept attribute

23 3NF Relations Emp3(emp-no, emp-name, dept) Dept3(dept, manager) EmpProj2(emp-no, proj-id, location, weeks-on-project) Proj2(proj-id, proj-start-date) 23 23

24 Third Normal Form Emp3 Proj3 Dept3 EmpProj3 Process 1NF - Remove repeating groups 2NF - Remove partial dependencies 3NF - Remove transitive dependencies 24 24