details on er and rm modeling - uppsala university€¦ · anne-kathrin peters- it - uu! 23.01.12 1...

23.01.12 1 Anne-Kathrin Peters- IT - UU!

Details on ER and RM modeling

Elmasri/Navathe ch 7,9 Padron-McCarthy/Risch ch 2-3"

Anne-Kathrin Peters

Department of Information Technology Uppsala University, Uppsala, Sweden


Content"

•  Last lecture (L2) and goal for L2 •  Comparison of assignment •  Translation of ER-model to RM – continued •  “Database in Action” BREAK •  EER and translation to RM


Last Lecture L2 Introduction to the Relational Model

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Conceptual Schema

Conceptual Design

Logical Design

Conceptual Model e.g. UML, ERM

Implementation Model (Data Model of DBMS here RM)

Logical Schema


Lessons learnt from ER modeling"

•  Be careful with attributes that are more like a relation: e.g.: “members” might sound like an attribute for the entity type “group”. NO- it is a relationship type (N:M). It needs to be implemented through an extra table!!!

•  An “artificial” attribute, e.g. id is quite convenient to use as a primary key •  Weak entities are not used so often (they don’t have a primary key) •  ER model is used to communicate with the user / to specify system

à Avoid too high complexity. Try to keep it simple J


From E-R to relational model"•  The basic procedure defines a set of relational schemas that represent entity

and relationship types in the E-R model. This model should further with integrity constraints. –  Primary keys allow entity types and relationship types to be expressed uniformly as tables

which represent the contents of the database. –  A database which conforms to an E-R diagram can be represented by a collection of tables. –  For each entity type and relationship type there is a unique table which is assigned the name

of the corresponding entity type or relationship type. –  Each table has a number of columns (generally corresponding to attributes), which have

unique names. –  Converting an E-R diagram to a table format is the basis for deriving a relational database

design from an E-R diagram.


Translation of ER modele.g. social network DB"

PROFILE(id, first name, last name, username, looking for, political views, photo_url)

PHOTO(url, description) PINBOARDENTRY(id, text, datetime) GROUP(id, description, creator_id) MEMBER(group_id, profile_id)


Translation of ER using Example"Weak entity types & owning relationship

relationship type with degree > 2

Multivalued attributes


1. Comparison"


Relational schemas for the example social network DB"

•  Schemas for the entity types PROFILE(id, first name, last name, username, looking for,

political views, photo_url) PHOTO(url, description, datetime) PINBOARDENTRY(id, text, datetime) GROUP(id, description, public, creator_id) LOCATION(id, street, nr, postal code, city, country) EVENT(id, description, datetime, public, creator_id,

location_id) •  Schemas for relationship types

MEMBER(group_id, profile_id, status) CONTAINS(pinentry_id, photo_url) ��LIVES(profile_id, location_id) INVITED(profile_id, event_id, answer) FRIENDWITH(profile1_id, profile2_id)


Mapping of n-ary Relationship Types"

1.  Make new table 2.  Decide on primary key

HAS(pinboardentry_id, group_id, creator_id) PINBOARDENTRY(id, text, datetime, group_id, creator_id)


Weak Entity types"•  Table with attributes as columns •  Primary key composed: candidate key of weak entity type and primary key

of owner entity type à “owner relationship” is mapped already

Comment(text, datetime, pinentryId, profileid)

Attention: This means that one user can’t comment with the same text on the same pinboard entry! So position is probably really a better partial key!


pk a1 pk k a2

R!

Translating entity types cont. . ."

a1

pk

a2

k 1! N!

•  Weak entity types - a weak entity type becomes a table that includes a column for the primary key of the identifying strong entity type .

E1


Mapping of multivalued Attributes"

•  Think of it as weak entity: –  Extra table –  Attribute A as one column –  Primary key:

•  Attribute A •  Primary key of owning entity

bands profile likes n 1name

bands(name, pId)


Relational schemas for the example social network DB"

•  Schemas for the entity types PROFILE(id, first name, last name, username, looking for,

political views, photo_url) BANDS(name, p_id) PHOTO(url, description, datetime) PINBOARDENTRY(id, text, datetime) COMMENT(position, datetime, pinboardentry_id, profile_id) GROUP(id, description, public, creator_id) LOCATION(id, street, nr, postal code, city, country) EVENT(id, description, datetime, public, creator_id,

location_id) •  Schemas for relationship types

MEMBER(group_id, profile_id, status) CONTAINS(pinentry_id, photo_url) ��GROUPHASPINENTRIES(pindentry_id, group_id, creator_id) PROFILEHASPINENTRIES(pindentry_id, adressee_id, creator_id) LIVES(profile_id, location_id) INVITED(profile_id, event_id, answer) FRIENDWITH(profile1_id, profile2_id)


Summary of Translations"ERM RM (see slides for details!)

Entity type Relation (Table) Simple Attributes Attribute in Relation (Column) Candidate key attribute(s)

Choose one as primary key (if more than one)

Composite Attributes Set of simple component attributes (columns) Multivalued Attributes Extra relation (“treat as weak entity type”) n:m Relation Relation (table), primary key: 2 foreign keys 1:n Relation Either include in “n-Entity”, else extra relation with

primary key: key of “n-Entity” 1:1Relation 3 options: 1. merge table with table of entity E1, E2,

2. extra table, 3. one table for E1, E2, R n-ary Relation Extra table (, merge?)


Use of Databases in WWW"

Source: Ludwig-Maximilians-University, Munich, Germany

JavaScript, JavaApplets PHP, Java Servlets, JSP

Execution client side Execution server side


Integrity constraints

1.  Domain constraint –  Values for attribute A shall be atomic values from dom(A)

2.  Key constraint –  candidate keys for a relation must be unique

3.  Entity integrity constraint –  no primary key is allowed to have a null value

4.  Referential integrity constraint –  Foreign Keys must refer to an existing tuple

5.  Semantic integrity constraint –  e.g. “an employee’s total work time per week can not exceed

40 hours for all projects taken all together”


Letʼs get a bit deeper intoDB design…"


ER model transf. cont. ..."

Replacing relationships with degree>2 with an entity type and binary relationships.

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cardinality/participation vs. min-max"

part_of!PRODUCT DETAIL 1 N

part_of!PRODUCT DETAIL (0,N) (1,1)


Extended Entity-Relationship (EER) modeling"

•  EER (extended or enhanced ER) –  Specialization / Generalization

•  Superclass / subclass •  supertype/subtype,

•  is-a relationship •  constraints

–  Aggregation (whole/part or part-of relationship) –  Union types (category)


EER diagram notation for specialization and generalization(Elmasri/Navathe Figure 7.19)!

partial/total (similar to ERM)

Disjunkt / Overlapping (à)

Type inheritance through: entity of subclass is also member of superclass!

Constraints on membership, e.g. through attribute “job_type”


Overlapping (nondisjoint) subclasses (Elmasri/Navathe Figure 7.23)"

Disjunkt / Overlapping (à)


Generalization of subclasses

(Elmasri/Navathe Figure 7.21)"


Summary Subclasses, superclasses & inheritance"

•  Two approaches for creating superclass/subclass relationships –  Specialization of superclass into subclasses –  Generalization of subclasses into a superclass

•  Entity of subclass is also entity of superclass •  Type inheritance: Type of entity- attributes and relationship •  Characteristics of specialization & generalization

–  Disjoint (d) vs. Overlapping (o) –  Completeness: Total vs. Partial


How to translate?"

Options: 1.  Multiple-Relation:

EMPLOYEE(Ssn,Fname,LBirthDate,…) TECHNICIAN(Ssn,Tgrade) SECRETARY(Ssn,TypingSpeed) ENGINEER(Ssn,EngType)

2.  Single Relation: ENGINEER(Ssn,Fname,Minit,Lname, BirthDate,Address,TypingSpeed, Tgrade,EngType)


Translation of Specialization / Generalization"

Given m Subclasses {S1, …, Sm} and (generalized) superclass C with attributes Attrs(C)={k,a1,…,an}, k primary key (PK(C)=k). à  Options for translation: 1.  Multiple relations, Superclasses and subclasses:

Relation for C, Attrs(C)={k,a1,…,an} and relation Li for each subclass with Attrs(Li)={k}∪ {attributes of Si}, PK(Li)=k (1≤i≤m)

2.  Multiple relations, subclasses only: Relation Li for each subclass Si , 1≤i≤m, Attrs(Li)={k,a1,…,an}∪ {attributes of Si}, PK(Li)=k (only for total subclass specialization)

3.  Single relation with one type attribute: Single Relation L, Attrs(Li)={k,a1,…,an}∪{attributes of S1}∪ … ∪ , {attributes of Sm}∪{t}, t type attribute, PK(Li)=k (for disjoint subclasses)

4.  Single relation with multiple type attributes: Single Relation L, Attrs(Li)={k,a1,…,an}∪{attributes of S1}∪ … ∪ , {attributes of Sm}∪{t1, t2… , tm} boolean type attributes, PK(Li)=k (for overlapping subclasses)


Multiple distinct superclass relationships"


Union / Category (Elmasri/Navathe

Figure 7.28)"


Summary & translation"

•  Union: –  Union of different entity types (instead of intersection of different entity types) –  Partial or total

•  Translation:


Representation of aggregation in ER notation"•  3 cases:

1.  Aggregation of attribute values to form object 2.  Aggregation of attribute to form a relationship 3.  Particular relationship “part-of” / “is-component-of” between primitive

objects •  No explicit representation of aggregation in ER notation��

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Representation of aggregation in

ER notation


3 cases: 1. Aggregation of attribute

values to form object 2. Aggregation of attribute to

form a relationship 3. Particular relationship

“part-of” / “is-component-of” between primitive objects


A UML conceptual schema (Elmasri/Navathe Figure. 9.1)"

Further information: www.uml.org


Specialization/generalization in UML


Further information: www.uml.org


Alternative diagrammatic

notation for ER/EER

(Elmasri/Navathe Figure A.1)"

details on er and rm modeling - uppsala university€¦ · anne-kathrin peters- it - uu! 23.01.12 1...

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