Modeling Your Data Chapter 2

Part II Discussion of the Model: Good Design/ Bad Design?

Design : The Obvious ! – Use meaningful and descriptive names (it’s for the human after all) – Keep as simple as possible, and relevant to the application at hand – Avoid redundant constructs – Express all constraints, if possible, as then the DBMS will help you to enforce them

Conceptual Design Using ER Model Design choices: – Should a concept be modeled as an entity or an attribute? – Should a concept be modeled as an entity or a relationship? – Identifying relationships: Binary or ternary? Aggregation? Constraints in the ER Model: – A lot of data semantics can (and should) be captured. – But some constraints cannot be captured in ER diagrams.

Entity vs. Attribute Should address be an attribute of Employees or an entity (connected to Employees by a relationship)?

Entity vs. Attribute Should address be an attribute of Employees or an entity (connected to Employees by a relationship)? Depends upon the use we want to make of address information and its semantics : If we have several addresses per employee, address must be an entity (since attributes cannot be set-valued). If the structure (city, street, etc.) is important, e.g., we want to retrieve employees in a given city, address must be modeled as an entity (since attribute values are atomic).

Entity vs. Attribute Reminder : Do not introduce un-necessary entities (and complexity) if not needed for your application !

Entity vs. Attribute name Employees ssn lot Works_In4 from to dname budget did Departments

Entity vs. Attribute Does Works_In4 allow an employee to work in a department for two or more periods??? name Employees ssn lot Works_In4 from to dname budget did Departments

Entity vs. Attribute (Contd.) Works_In4 does not allow an employee to work in a department for two or more periods  no multi-valued attributes in ER ! name Employees ssn lot Works_In4 from to dname budget did Departments

Entity vs. Attribute (Contd.) Similar to the problem of wanting to record several addresses for an employee: We want to record several values of the descriptive attributes for each instance of this relationship. Accomplished by introducing new entity set, Duration. name Employees ssn lot Works_In4 from to dname budget did Departments Works_In4 does not allow an employee to work in a department for two or more periods. What do ?

Entity vs. Attribute name Employees ssn lot Works_In4 from to dname budget did Departments dname budget did name Departments ssn lot Employees Works_In4 Duration from to

Entity vs. Relationship? Manages2 name dname budget did Employees Departments ssn lot dbudget since

Entity vs. Relationship What if a manager gets a separate discretionary budget for each dept ? What if a manager gets a discretionary budget that covers all managed depts? Manages2 name dname budget did Employees Departments ssn lot dbudget since

Entity vs. Relationship ER diagram OK if a manager gets a separate discretionary budget for each dept. What if a manager gets a discretionary budget that covers all managed depts? – Redundancy: dbudget stored for each dept managed by manager. – Misleading: Suggests dbudget associated with department-mgr combination. Manages2 name dname budget did Employees Departments ssn lot dbudget since

Entity vs. Relationship Discretion -ary budget of manager that covers all managed depts? Manages2 name dname budget did Employees Departments ssn lot dbudget since dname budget did Departments Manages2 Employees name ssn lot since Managersdbudget ISA This fixes the problem!

Binary vs. Ternary Relationships age pname Dependents Covers name Employees ssn lot Policies policyid cost

Binary vs. Ternary Relationships age pname Dependents Covers name Employees ssn lot Policies policyid cost  What if each policy is owned by just 1 employee?  What if each dependent should be tied to only 1 covering policy?

Binary vs. Ternary Relationships What do additional constraints in 2nd diagram? age pname Dependents Covers name Employees ssn lot Policies policyid cost Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Bad design! Better design?

Binary vs. Ternary Relationships age pname Dependents Covers name Employees ssn lot Policies policyid cost Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Bad design Even Better Design

Binary vs. Ternary Relationships If each policy is owned by just 1 employee, and each dependent is tied to the covering policy, first diagram is inaccurate. What are the additional constraints in the 2nd diagram? age pname Dependents Covers name Employees ssn lot Policies policyid cost Beneficiary age pname Dependents policyid cost Policies Purchaser name Employees ssn lot Bad design Better design

Binary vs. Ternary Relationships Previous example illustrated when two binary relationships better than one ternary relationship. How about ternary relation Contracts : –entity sets Parts, Departments and Suppliers, –and has descriptive attribute qty. pnum Parts contract Suppliers num Department D-id Qty

Binary vs. Ternary Relationships Ternary relation Contracts relates entity sets Parts, Departments and Suppliers, and has attribute qty. What about following binary relationships : –S “can-supply” P, – D “needs” P, and – D “deals-with” S No combination of binary relationships is an adequate substitute: – Together 3 binary relationships don’t imply that D has agreed to buy P from S. – Also, how could we record qty?

Summary of ER There are often many ways to model a given scenario! Not one correct ER design Analyzing alternatives can be tricky, especially for a large enterprise. Common choices include: – Entity vs. attribute, entity vs. relationship, binary or n-ary relationship, whether or not to use ISA hierarchies, and whether or not to use aggregation. Ensuring good database design: - Resulting relational schema should be analyzed and refined further. FD information and normalization techniques useful.