1. Database Design – Lecture 7
Converting a Logical ERD to a Relational Model

2. Lecture Objectives
How to convert entities and their relationships from the logical model to the relational model
Refining the key structure of an entity
Refining relationships between entities based on Business Rules

3. Converting Entities and Attributes
Create a separate table for each entity and consider all the attributes defined as the table columns
Verify PK defined in logical model – is it appropriate for table or does another unique (generic) PK need to be defined
Remove derived attributes

4. Converting Entities and Attributes
Convert composite attributes to atomic attributes
i.e. NAME: convert to FIRST_NAME, LAST_NAME, INITIAL
i.e. ADDRESS: convert to STREET, CITY, PROVINCE, POSTAL_CODE

5. Converting Entities and Attributes
Convert multi-valued attributes
Approach 1:
Within original entity, create several new attributes, one for each of the original multi-valued attribute’s components
Can lead to major structural problems in table
Approach 2:
Create a new entity composed of original multi-valued attribute’s components
A 1:M dependent relationship to original table will exist
Approach 3:
Create a new entity composed of multi-valued attribute’s components
A M:N relationship to original table will exist

6. Converting Entities and Attributes
Consider an employee who has multiple skills
Approach 1:
Create attributes to hold the skill information
What if an employee has more than 3 skills?

7. Converting Entities and Attributes
Consider an employee who has multiple skills
Approach 2:
Create a new entity to hold the attribute’s components
Existent dependent because these skills are for this employee and are of no value without the employee

8. Converting Entities and Attributes
What if other employees can have the same skills?
Approach 3:
Create a new entity to hold the attribute’s components
Results in a M:N relationship between EMPLOYEE and SKILL entities which will require a bridge table to be created

9. Converting Relationships
For 1:M connectivity, take the PK of the ‘1’ table and make it an FK in the ‘M’ table
For M:N connectivity, create a bridge table
Include the PK from each of the original tables as composite PK in the bridge table. These will also be FKs in the bridge table
Add additional attributes to the bridge table as required
For Strong – Weak Entity relationships, take the PK of the Strong Entity and include it as part of the PK of the Weak Entity. It will also be an FK in the Weak Entity

10. Example Conversion
Given the following ERD, convert to a relational structure using the techniques provided
**Note: this ERD is slightly different than last class. Changed to provide more attributes to demonstrate converting to relational tables.

11. Example Conversion Resulting Tables Might Look Like:
CUSTOMER (CUST_NUMBER (PK), CUST_FNAME, CUST_LNAME, CUST_INITIAL, STREET_ADDRESS, CITY, PROVINCE, POSTAL_CODE, HOME_PHONE, DOB)
LIBRARIAN (EMPL_ID (PK), LIBR_FNAME, LIBR_LNAME, LIBR_INITIAL)
BORROWING_TRANSACTION (TRANS_ID (PK), CUST_ID (FK), EMPL_ID (FK), DATE)
BORROWING_DETAIL (TRANS_ID (PK, FK), LINE_NUMBER (PK), BOOK_ID (FK))
CATEGORY (CAT_ID (PK), NAME)
BOOK (BOOK_ID (PK), CAT_ID (FK), TITLE, AUTHOR, DESCRIPTION, ISBN)