Entity-Relationship Models Biological Database Systems Boston University Bioinformatics Graduate Program

Entity-Relationship Models

Biological Database Systems

Boston University

Bioinformatics Graduate Program

Course BE768

Spring 2003

Biological Database Systems, BE768 1

Overview of Database Design

• Conceptual design : ( ER Model is used at this stage.)

• What are the entities and relationships in the enterprise?

•

•

• What information about these entities and relationships should we store in the database?

What are the integrity constraints or business rules that hold?

A database `schema’ in the ER Model can be represented pictorially ( ER diagrams ).

• Can map an ER diagram into a relational schema.


ER Model Basics

• Entity: Real-world object distinguishable from other objects. An entity is described (in DB) using a set of attributes .

• Entity Set : A collection of similar entities. E.g., all employees.

• All entities in an entity set have the same set of attributes.

(Until we consider ISA hierarchies, anyway!)

•

• Each entity set has a key .

Each attribute has a domain .

ssn name

Employees lot


Database Models: Entity vs. Physical

Entity Model

–What is an entity?

– An entity is any distinguishable object that is to be stored in the database. Tissue and sequence are examples of entities.

Entities are linked by relationships. The fact that a sequence was obtained from a particular tissue is a relationship. It is possible that a relationship can also be an entity if it is to be stored.

–Entities have properties that describe the entity. The sequence length is a property of the sequence (though we prefer not to store a property that can be derived from another: sequence length can be directly measured from the sequence itself). A tissue name is a property of tissue.


Entities, continued

There are four basic types of relationships (also referred to as the cardinality) between entities:

– 1-1

– 1-many

– Many-1

– Many-many


Physical Model

- The physical model dwells on the details of the tables themselves e.g. columns, indexes, and referential integrity. For the purposes of the development of exploratory databases in bioinformatics, there are only two types of tables to worry about: object tables and relationship tables. The object tables hold information about “basic” entities (e.g. sequences or tissue) and the relationship tables hold the information about how the objects are related (e.g. which sequence was found in which tissue).


ER Model Basics cont.

• Relationship : Association among two or more entities. E.g., Attishoo works in Pharmacy department.

• Relationship Set : Collection of similar relationships.

• An n-ary relationship set R relates n entity sets E1 ... En; each relationship in R involves entities e1 E1, ..., en En

• Same entity set could participate in different relationship sets, or in different “roles” in same set.

name ssn lot ssn name lot

Employees since did dname budget

Works_In Departments

Biological Database Systems, BE768

Employees supervisor

Reports_To

7

Key Constraints

• Consider Works_In:

An employee can work in many departments; a dept can have many employees.

• In contrast, each dept has at most one manager, according to the key constraint on

Manages.

ssn name

Employees lot since

Manages did dname budget

Departments


1-to-1 1-to Many Many-to-1 Many-to-Many

8

Participation Constraints

• Does every department have a manager?

• If so, this is a participation constraint : the participation of

Departments in Manages is said to be total (vs. partial ) .

• Every did value in Departments table must appear in a row of the Manages table (with a non-null ssn value!) ssn

Employees lot

Manages

Works_In since


Departments

9

Weak Entities

• A weak entity can be identified uniquely only by considering the primary key of another ( owner ) entity.

• Owner entity set and weak entity set must participate in a one-to-many relationship set (one owner, many weak entities).

• Weak entity set must have total participation in this identifying relationship set. name ssn

Employees lot cost

Policy pname age

Dependents


ISA (‘is a’) hierarchies name ssn lot

Employees

•

As in C++, or other PLs, attributes are inherited.

•

If we declare A ISA B, every

A entity is also considered to be a B entity. hourly_wages hours_worked

Hourly_Emps

ISA contractid

Contract_Emps

• Overlap constraints : Can Joe be an Hourly_Emps as well as a

Contract_Emps entity? ( Allowed/disallowed )

• Covering constraints : Does every Employees entity also have to be an

Hourly_Emps or a Contract_Emps entity?

(Yes/no)

• Reasons for using ISA:

• To add descriptive attributes specific to a subclass.

• To identify entitities that participate in a relationship.


Aggregation

• Used when we have to model a relationship involving (entitity sets and) a relationship set .

• Aggregation allows us to treat a relationship set as an entity set for purposes of participation in (other) relationships.

ssn name

Employees

Monitors lot until pid started_on pbudget since did dname budget

Projects

Sponsors Departments

•

•

 Aggregation vs. ternary relationship:

Monitors is a distinct relationship, with a descriptive attribute.

Also, can say that each sponsorship is monitored by at most one employee.


Conceptual Design Using the 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)?

• Depends upon the use we want to make of address information, and the semantics of the data:

• 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 cont.

•

Works_In4 does not allow an employee to work in a department for two or more periods.

•

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. ssn name

Employees lot from to

Works_In4 did dname budget

Departments ssn name

Employees lot

Works_In4 did dname budget

Departments from Duration to


Entity vs. Relationship

• First 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.

ssn ssn name

Employees name

Employees lot lot since dbudget

Manages2 since

Manages2 did did

ISA dname budget

Departments dname budget

Departments

Managers dbudget

This fixes the problem!


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?

ssn name

Employees

Bad design ssn name

Employees lot lot policyid

Covers

Policies cost pname age

Dependents pname age

Dependents

Purchaser

Beneficiary

Better design policyid

Policies cost


Binary vs. Ternary Relationships

• Previous example illustrated a case when two binary relationships were better than one ternary relationship.

• An example in the other direction: a ternary relation Contracts relates entity sets Parts, Departments and Suppliers , and has descriptive attribute qty . No combination of binary relationships is an adequate substitute:

•

S “can-supply” P, D “needs” P, and D “deals-with” S does not imply that D has agreed to buy P from S.

• How do we record qty ?


Summary of Conceptual Design

• Conceptual design follows requirements analysis ,

• Yields a high-level description of data to be stored

• ER model popular for conceptual design

• Constructs are expressive, close to the way people think about their applications.

• Basic constructs: entities , relationships , and attributes (of entities and relationships).

• Some additional constructs: weak entities , ISA hierarchies , and aggregation .

• Note: There are many variations on ER model.


Summary of ER

• Several kinds of integrity constraints can be expressed in the ER model: key constraints , participation constraints , and overlap/covering constraints for ISA hierarchies. Some foreign key constraints are also implicit in the definition of a relationship set.

•

• Some constraints (notably, functional dependencies ) cannot be expressed in the ER model.

Constraints play an important role in determining the best database design for an enterprise.


Summary of ER cont.

• ER design is subjective . There are often many ways to model a given scenario! 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 are especially useful.


ER Example


ER to Tables


ER to tables cont.












ER


ER


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