CSc340 5b1 Database Design and the E-R Model Chapter 7 [1 of 2] Overview of the Design Process The...
Transcript of CSc340 5b1 Database Design and the E-R Model Chapter 7 [1 of 2] Overview of the Design Process The...
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CSc340 5b 1
Database Design and the E-R Model
Chapter 7 [1 of 2]
Overview of the Design ProcessThe Entity-Relationship Model
ConstraintsRemoving Redundant Attributes in Entity Sets
Entity-Relationship Diagrams
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"BrainStorming"(Mind Mapping)
Brainstorming can be an effective way to generate lots of ideas on a specific issue and then determine which idea – or ideas – is the best solution.
Pick an off-the-wall topic, say, Circuses What is involved (entities)?
performers, resources Refine
human vs. animal performers, equipment vs. arena
CSc340 5b 2
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Modeling A database can be modeled as:
a collection of entities, relationship among entities.
An entity is an object that exists and is distinguishable from other objects.
Example: specific person, company, event, plant
Entities have attributes Example: people have names and addresses
An entity set is a set of entities of the same type that share the same properties.
Example: set of all persons, companies, trees, holidays
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Entity Sets Entity Sets instructor instructor and and studentstudent
instructor_ID instructor_name student-ID student_name
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Relationship Sets
A relationship is an association among several entitiesExample:
44553 (Peltier) advisor 22222 (Einstein) student entity relationship instructor entity
A relationship set is a mathematical relation among n 2 entities, each taken from entity sets
{(e1, e2, … en) | e1 E1, e2 E2, …, en En}
where (e1, e2, …, en) is a relationship Example:
(44553,22222) advisor
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Relationship Set advisor
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Relationship Sets (Cont.) An attribute can also be property of a relationship set. For instance, the advisor relationship set between entity sets
instructor and student may have the attribute date which tracks when the student started being associated with the advisor
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Degree of a Relationship Set
binary relationship involve two entity sets (or degree two). most relationship sets in a database system are
binary. Relationships between more than two
entity sets are rare. (More on this later.) Example: students work on research projects
under the guidance of an instructor. relationship proj_guide is a ternary relationship
between instructor, student, and project
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Attributes An entity is represented by a set of attributes, that is
descriptive properties possessed by all members of an entity set.
Example: instructor = (ID, name, street, city, salary )
course= (course_id, title, credits) Domain – the set of permitted values for each attribute Attribute types:
Simple and composite attributes. Single-valued and multivalued attributes
Example: multivalued attribute: phone_numbers Derived attributes
Can be computed from other attributes Example: age, given date_of_birth
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Composite Attributes
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Mapping Cardinality Constraints
Express the number of entities to which another entity can be associated via a relationship set.
Most useful in describing binary relationship sets.
For a binary relationship set the mapping cardinality must be one of the following types:
One to one One to many Many to one Many to many
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Mapping Cardinalities
One to one
One to many
Note: Some elements in A and B may not be mapped to any elements in the other set
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Mapping Cardinalities
Many to one
Many to many
Note: Some elements in A and B may not be mapped to any elements in the other set
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Keys
A super key of an entity set is a set of one or more attributes whose values uniquely determine each entity.
A candidate key of an entity set is a minimal super key
ID is candidate key of instructor course_id is candidate key of course
Although several candidate keys may exist, one of the candidate keys is selected to be the primary key.
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Keys for Relationship Sets
The combination of primary keys of the participating entity sets forms a super key of a relationship set.
(s_id, i_id) is the super key of advisor NOTE: this means a pair of entity sets can have at
most one relationship in a particular relationship set.
Example: if we wish to track multiple meeting dates between a student and her advisor, we cannot assume a relationship for each meeting. We can use a multivalued attribute though
Must consider the mapping cardinality of the relationship set when deciding what are the candidate keys
Need to consider semantics of relationship set in selecting the primary key in case of more than one candidate key
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Redundant Attributes
Suppose we have entity sets instructor, with attributes including dept_name departmentand a relationship inst_dept relating instructor and department
Attribute dept_name in entity instructor is redundant since there is an explicit relationship inst_dept which relates instructors to departments
The attribute replicates information present in the relationship, and should be removed from instructor
BUT: when converting back to tables, in some cases the attribute gets reintroduced, as we will see.
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E-R Diagrams
Rectangles represent entity sets. Diamonds represent relationship sets. Attributes listed inside entity rectangle Underline indicates primary key attributes
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Entity With Composite, Multivalued, and Derived Entity With Composite, Multivalued, and Derived AttributesAttributes
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Relationship Sets with Attributes
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Roles
Entity sets of a relationship need not be distinct Each occurrence of an entity set plays a “role” in the relationship
The labels “course_id” and “prereq_id” are called roles.
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Cardinality Constraints
We express cardinality constraints by drawing either a directed line (), signifying “one,” or an undirected line (—), signifying “many,” between the relationship set and the entity set.
One-to-one relationship: A student is associated with at most one instructor via the
relationship advisor A student is associated with at most one department via
stud_dept
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One-to-One Relationship
one-to-one relationship between an instructor and a student
an instructor is associated with at most one student via advisor
and a student is associated with at most one instructor via advisor
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One-to-Many Relationship
one-to-many relationship between an instructor and a student
an instructor is associated with several (including 0) students via advisor
a student is associated with at most one instructor via advisor,
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Many-to-One Relationships
In a many-to-one relationship between an instructor and a student,
an instructor is associated with at most one student via advisor,
and a student is associated with several (including 0) instructors via advisor
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Many-to-Many Relationship
An instructor is associated with several (possibly 0) students via advisor
A student is associated with several (possibly 0) instructors via advisor
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Participation of an Entity Set in a Relationship Set
Total participation (indicated by double line): every entity in the entity set participates in at least one relationship in the relationship set E.g., participation of section in sec_course is total
every section must have an associated course Partial participation: some entities may not participate in
any relationship in the relationship set Example: participation of instructor in advisor is partial
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Alternative Notation for Cardinality Limits
Cardinality limits can also express participation constraints
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E-R Diagram with a Ternary Relationship
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Cardinality Constraints on Ternary Relationship
We allow at most one arrow out of a ternary (or greater degree) relationship to indicate a cardinality constraint
E.g., an arrow from proj_guide to instructor indicates each student has at most one guide for a project
If there is more than one arrow, there are two ways of defining the meaning.
E.g., a ternary relationship R between A, B and C with arrows to B and C could mean
1. each A entity is associated with a unique entity from B and C or
2. each pair of entities from (A, B) is associated with a unique C entity, and each pair (A, C) is associated with a unique B
Each alternative has been used in different formalisms To avoid confusion we outlaw more than one arrow
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"BEER" Example
For the relationship Sells, we might have a relationship set like:
BAR BEERJoe’s Bar BudJoe’s Bar MillerSue’s Bar BudSue’s Bar Pete’s AleSue’s Bar Bud Lite
BAR BEERSells
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Avoiding Redundancy
Redundancy occurs when we say the same thing in two different ways.
Redundancy wastes space and (more importantly) encourages inconsistency. The two instances of the same fact may
become inconsistent if we change one and forget to change the other, related version.
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Example: Good
BEER MANF
name
This design gives the address of each manufacturer exactly once.Note that the attributes are pulled out of the entity type boxes for emphasis.
name addr
MadeBy
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Example: Bad
BEER MANF
name
This design states the manufacturer of a beer twice: as an attribute and as a related entity.
name
manf
addr
MadeBy
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Example: Bad
BEER
name
This design repeats the manufacturer’s address once for each beer; loses the address if there are temporarily no beers for a manufacturer.
manf manfAddr
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Entity Types Versus Attributes
An entity type should satisfy at least one of the following conditions:
It is more than the name of something; it has at least one non-identifier attribute.
-OR- It is the “many” in a many-one or
many-many relationship.
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Example: Good
BEER MANF
name
•MANF deserves to be an entity type because of the non-identifier attribute addr.•BEER deserves to be an entity type because it is the “many” of the many-one relationship MadeBy.
name addr
MadeBy
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Example: Bad
BEER MANF
name
Since the manufacturer is nothing but a name, and is not at the “many” end of any relationship, it should not be an entity type.
name
MadeBy
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Example: Good
BEER
name
There is no need to make the manufacturer an entity type if we record nothing about manufacturers besides their name.
manf
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Example
DRINKER addrname
BEER
manfname
BAR
name
license
addr
Note:license =beer, full,none
Sells
DrinksFrequents
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As Ternary Relationship
BAR BEER
DRINKER
name nameaddr manf
name addr
license
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Example
BAR BEER
price
Price is a function of both the bar and the beer,not of one alone. Attribute of relationship, not either entity!
Sells
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from Hannay Reels SALESPERSON entity calls on CUSTOMER entity SALES_CALL relationship has attributes:
date issues discussed personnel changes since last call etc.
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Weak Entity Sets
An entity set that does not have a primary key is referred to as a weak entity set.
The existence of a weak entity set depends on the existence of a identifying entity set
It must relate to the identifying entity set via a total, one-to-many relationship set from the identifying to the weak entity set
Identifying relationship depicted using a double diamond
The discriminator (or partial key) of a weak entity set is the set of attributes that distinguishes among all the entities of a weak entity set.
The primary key of a weak entity set is formed by the primary key of the strong entity set on which the weak entity set is existence dependent, plus the weak entity set’s discriminator.
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Weak Entity Sets (Cont.)
We underline the discriminator of a weak entity set with a dashed line.
We put the identifying relationship of a weak entity in a double diamond.
Primary key for section – (course_id, sec_id, semester, year)
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Weak Entity Sets (Cont.)
Note: the primary key of the strong entity set is not explicitly stored with the weak entity set, since it is implicit in the identifying relationship.
If course_id were explicitly stored, section could be made a strong entity, but then the relationship between section and course would be duplicated by an implicit relationship defined by the attribute course_id common to course and section
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Why Weak Entity Sets?
7.19 We can convert any weak entity set to a strong entity set by simply adding appropriate attributes. Why, then, do we have weak entity sets?
We want to avoid the data duplication and consequent possible inconsistencies caused by duplicating the key of the strong entity.
Weak entities reflect the logical structure of one entity being dependent on another entity.
Weak entities can be deleted automatically when their strong entity is deleted.
CSc340 5b 46
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E-R Diagram
for a University Enterprise
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CSc340 5b 48
Homework/Project
Homework due Next Class: 6.1, 6.2, 6.5, 6.6, 6.7, 6.8
No Homework due in One Week
Project Create an E-R Diagram for your enterprise NOT the same as Schema Diagram!
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CSc340 5b 49
In-Class Exercises
7.15 Construct an E-R diagram for a hospital with a set of patients and a set of medical doctors. Associate with each patient a log of the various tests and examinations conducted.
7.17 Design an E-R diagram to keep track of games played, scores of each game, players in each game, and player statistics for each game for all teams in a league. Summary statistics should be modeled as derived attributes. (cf. HW 7.3)