LOGICAL DATABASE DESIGN

• Starting Point --> Local Conceptual Models• Database design methodology: conceptual,

logical and physical database design.• Logical database Design:

– The process of constructing a model of information used in an enterprise based on a specific data model, but independent of a particular DBMS or other physical considerations.

• Step2: Build and validate local logical data model for each user view.

• Step3: Build and validate global logical data model

Step2: Build and Validate Local

Logical Data model for each user view. • Step2.1: Map local conceptual data model to local

logical data model.

• Step2.2: Derive relations from logical data model.

• Step2.3: Validate model using Normalization.

• Step2.4: Validate model against user transactions

• Step2.5: Draw E-R Diagram.

• Step2.6: Define integrity constraints

• Step2.7: Review local logical data model with user.

Step2.1: Map local conceptual data model to local logical data model.

• Objectives:– Remove M:N relationships.– Remove complex relationships.– Remove recursive relationships– Remove relationships with attributes– Remove multi-valued attributes– Re-examine 1:1 relationships– Remove redundant relationships.

Step2.2: Derive relations from logical data model• Objective: To derive relations from the local logical data model• Relationship:

– primary key/ foreign key mechanism– identify the ‘parent’ and ‘child’ entities

• Strong entity types– Staff (Staff_No, Fname, Lname, Street, City, Postcode, Position, Sex, Salary)

– PrimaryKey Staff_No

• Weak entity types– include a foreign key - - - of the owner entity

– the primary key of a weak entity is partially or fully derived from the owner entity.

• Next-of-Kin (Staff_No,Name, Address, Tel_No, Relationship)

• Primary Key: Staff_No, Name

• Foreign Key: Staff_No, references Staff(Staff_No)

Step2.3: Validate model using Normalization• Objective: To validate a local logical data model using the

technique of normalization.– Organizes the data according to functional dependencies– A normalized relationship is robust and free of update anomalies– Normalization provides a flexible database design that can easily be

extended

• First Normal Form (1NF), removes repeating relationship

• Second Normal Form (2NF), removes partial dependencies on the primary key

• Third Normal Form (3NF), removes transitive dependencies on the primary key

Step2.4: Validate model against user transactions

• Objective

– To ensure that the local logical data model supports the transactions that are required by the user views.

– ER diagram, data dictionary, primary key/ foreign key links shown in the relations

– Likely that we have omitted an entity, a relationship or an attribute.

• (a) Insert details for a new member of staff– First check that the new staff no does not already exist -->

Prohibit the insertion; otherwise insert the new staff details..

• (b) Delete details of a given staff no– Search for the given staff no, if not found --> user error,

otherwise delete the tuple.

Staff PropertyManages

Staff_No Property_No

1 M

Step2.5: Draw E-R Diagram

• Objective– To identify the final ER diagram that is a local

logical representation of the given data given in a user’s view of the enterprise.

Step2.6: Define integrity constraints

• Objective: to define the entity constraints given in a user’s view of the enterprise.

• Five types of integrity constraints– Required data– Attribute domain constraints– Entity integrity– Referential Integrity– Enterprise constraints

• Required data – Not allowed to hold nulls

• e.g.: every member of staff must have an associated position

• Attribute Domain Constraints– e.g.: ‘M’ or ‘F’ for Gender

• Entity Integrity– Primary key of entity cannot hold nulls

• Referential Integrity– The foreign key contains a value that matches with an

existing occurrence in the parent table.

• Case1: Insert occurrence into child relation (Property)– To ensure referential integrity Staff_No -- property occurrence.

• Case2: Delete occurrence from child relation– No problem as referential integrity is not affected.

• Case3: Update foreign key of a child occurrence– Ensure referential integrity

• Case4: Insert occurrence into parent table – Does not cause a problem for referential integrity . Parent without any

children . Staff member without a property to manage.

• Case5: Delete occurrence from Parent relation (Staff)– referential integrity is lost if there exists a child's

occurrence.– Several strategies

• NO ACTION: Prevent deletion from parent relation, if any child reference.

• CASCADE: When the parent occurrence is deleted, delete any child occurrences (not wise)

• SET NULL: When the parent occurrence is deleted the child occurrences are set to NULL

• SET DEFAULT: The foreign key values in all child occurrences should be set to default values.

– Eg: Handled by another (default) member of staff -- Manager

• NO CHECK: When a parent occurrence is deleted do nothing to ensure referential integrity exists.

• Case6: Update the primary key of parent occurrences (Staff)– If the primary key value of the parent occurrence is

updated, referential integrity is lost if there exists a child occurrence referencing the old primary key value.

• Enterprise Constraints– Called business rules.

• Eg: No member of staff can manage more than 10 properties at a time

• Document all Integrity Constraints

Step2.6: Review local logical data model with user

• Objective– To ensure that the logical data model is a true

representation of the user’s view.

• Relationship between the logical data model and data flow diagram – Each datastore should represent a whole number of

entity types– Attributes on data flows should belong to entity

types

Step3: Build and Validate Global Logical data Model

• Objective– To combine the individual data logical models into a single

global logical data model that can be used to represent the part of the enterprise that we are interested in modeling.

• Step3.3: Merge local logical data models into global model

• Step3.2: Validate global logical data model

• Step3.3: Check for future growth

• Step3.4: Draw final ER diagram

• Step3.5: Review global logical data model with users.

Step3.1: Merge local logical data models into global model• Objective

– To merge the individual local logical data models into a single global logical data model of the enterprise.

• Typical tasks• Review the names of entities and their primary keys• Review the names of relationships• Merge entities form local views• Include (without merging) entities unique to each user view.• Merge relationships from the local views• Include (without merging) entities unique to each local view.• Check for missing entities or relationships• Check foreign keys• Check integrity constraints• Draw the global logical data model• Update the documentation

Staff (Staff_No, Name, Position, Sex, Salary, Branch_No)

Primary Key: Staff_No

Foreign Key: Branch_No references Branch(Branch_No)

Staff (Staff_No, Fname, Lname, Address, Branch_No)

Primary Key: Staff_No

Foreign Key: Branch_No references Branch(Branch_No)

Staff (Staff_No, Fname, Lname, Address, Position, Sex, Salary, Branch_No)

Primary Key: Staff_No

Foreign Key: Branch_No references Branch(Branch_No)

View1

View2

Global View

Merging

• Merge entities from the local views– Merge entities with the same name and primary key

– Merge entities with the same name using different primary keys

– Merge entities with different names using different primary keys

View1

Staff (Staff_No, Name, Position, Sex, Salary, Branch_No)

Primary Key: Name

Alternate Key: Staff_No

Foreign Key: Branch_No references Branch(Branch_No)

View2:

Staff (Staff_No, Fname, Lname, Address, Branch_No)

Primary Key: Staff_No

Alternate Keys: Fname, LName

Foreign Key: Branch_No references Branch(Branch_No)

Merging equivalent entities using different primary keys

• Step3.2: Validate Global Logical Data Model

• Step3.3: Check for future growth

• Step3.4: Draw final ER diagram

• Step3.5: Review global logical data model with users.