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

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

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

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

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

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?

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

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

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

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.

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)