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 Staff_No Property_No M 1 Staff Manages Property (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.

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

Attribute Domain Constraints Entity Integrity Referential Integrity 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

View1 View2 Global View Merging 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) Staff (Staff_No, Fname, Lname, Address, Position, Sex, Salary, Branch_No) View1 View2 Global View

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

Merging equivalent entities 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

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.