Database Management

2. course

Reminder

• Concept of DBMSs:– Gathering all the data and correspondence into one DB– Answer the question with the help of this DB

• 3 levels of a DB– Conceptual model: A world described by the DB– Implementation/representation model: a model

understandable for the DBMS (structured records, tables, fields, etc.)

– Physical model: DBMS implemented on the computer (files, programs)

Structure of the DBMS

• The user has permission for the smaller part of the DB: View

Conceptual model

Implementation model

Physical model

View1 View2 View3

Relational data model

• Relation:table+constraints

• Column headers:attribute/domain

• Rows:data records/tuples

• Database:Set of tables

Relationships

• 1:1

• 1:N

• N:M

Person Owns Car

Actor Acts Play

• Superkey/Key/Foreign key• Anomalies

– Insertion: superkey needs too much data– Update: Redundancy– Deletion: Different objects stored together

Normalization

• If a relation is in 0 NF (can be put in tables) and does not contain multiple field then it is in at least 1 NF

• If a relation is in 1 NF and does not contain partial functional dependency then it is in at least 2 NF

• If a relation is in 2 NF and does not contain transitive functional dependency then it is in at least 3 NF

3 NF vs. BCNF

• A 3 NF is not in BCNF if– There are several possible keys,– these keys are composite, and– there is a common attribute in the keys

Decomposition has to be…???

Decomposition

• Has to be– Lossless: By uniting/jointing the decomposed tables,

the original tables before normalization can be created

– Preserve dependencies: After decomposition, the originial dependencies can be infered from the new relation’s dependencies.

• Closure of an attribute set• Armstrong axioms• How to check?

Today

• Administration• Concurrent processing• Entity-relationship diagrams

Tasks of the database supervisor

• Administration• Protect the systems• Protect the users• Design• Keep in touch with the users• Maintain, save, backup

Logical independence

Conceptual model

Implementation model

Physical model

View1 View2 View3

Physical independence

Conceptual model

Implementation model

Physical model

View1 View2 View3

Transaction

• Unit• Basic operation in the DB• Has to preserve the consistency of the DB

Concurrent processing

• Basic requirement for fast execution• Inconsistency• User is independent

Steps of record processing

1. Search2. Import to the workspace of the user3. Process, update4. Write back to database

Wait-for graph

• Ek(Ti, Tj): Ti gets mean k before Tj

• Ti: ith transaction

Rollback

• Rollback journal• Difference table

Log

• Atomicy• WAL protocol• Crash?

Logfile

• Content• Concatenated or rolled back• Archived• Invisible

Tasks of the DB administrator

• Data independence• Comfort• Security, secrecy, accuracy• Concurrent processes• Reduce response time• Develop• Expenses

Structure of the DBMS

• LayersQuery optimization and

-execution

Relational operators

Files and permissions

Buffering

Handling storage

DB

Handling concurrency andrecovery control is takeninto consideration

Entity relationship (ER) diagrams

Steps of designing a database

• Survey the needs• Design the conceptual model• Create the implementation model• Design the physical model

Steps of the conceptual designing

1. Define the entities and relationships2. Define the info about the entites3. Define the constraints

Elements of the ER model

• Entities– Properties– Key

• Relationships– 1:1– 1:N– N:M

Entity set

• Set of similar entities• Key• Codomain

Notation

• Entity set:

• Attribute:

• Key attribute:

Student

sidname

avg

Relationships and relationship sets

• Relationship:– David works in the purchasing department

• Relationship set:– n-ary relation:, elements of the relation: ,

Notation

• Relationship set:

• M:N relationship:

• N:1 relationship:

lot

dname

budgetdid

sincename

Works_In DepartmentsEmployees

ssn

Reports_To

lot

name

Employees

super-visor

ssn

Types of relationships

lot

dname

budgetdid

since

name

Works_In DepartmentsEmployees

ssn

dname

budgetdid

since

lot

name

ssn

Employees DepartmentsManages

ER model of a university

• What assumptions are there?

• How to check correctness? (Ask questions!)

Students

Professor teaches

Department

faculty

major offers

Courses

enrollmentadvisor

Practice

• Can a student have more than one major?

• Can a student have more than one advisor?

• Can a professor work in more than one department?

• Can a course have more than one teacher?

• Can a department be without teacher?

Students

Professor teaches

Department

faculty

major offers

Courses

enrollmentadvisor

Constraints

• Does all the departments have a manager?– Participation constraint– Bold or double line

lot

name dnamebudgetdid

sincename dname

budgetdid

since

Manages

since

DepartmentsEmployees

ssn

Works_In

Weak entity set

• Attributes of the entity set are not enough– Notation: double or bold line for the entity set

Studios(name, addr)Crews (number, studioName)Unit-of (number, studioName, name)name=studioName!!!

Example

• Disney crew #3 is one of the crews of the Disney studio

• Crews: (Disney-crew-#3, Disney)• Unit-of: (3, Disney, Disney)

• Solution:• Merge to: (3, Disney)• Same as Crews leave it for good

Subclass, inheritance (ISA)

Length Title Year

WeaponMovies

CriminalCartoon

isa isa

isa isa

Criminal cartoon

Voice

Actor

• Special properties can be given• Different relationships with other entities• Inheritance• Overlap constraint• Covering constraint

Decisions in design

• Entity or property?• Entity or relationship?• Relationship: binary or ternary?• Constraints

Entity or attribute?

• Should the address of an employee be an entity or an attribute of the worker table?

• Attribute:– Simple– Atomic

Example

name

Employees

ssn lot

Works_In

from to

dname

budgetdid

Departments

dname

budgetdidname

Departments

ssn lot

Employees Works_In

Durationfrom to

Entity or relationship

Manages2

name dnamebudgetdid

Employees Departments

ssn lot

dbudgetsince

dname

budgetdid

DepartmentsManages2

Employees

name

ssn lot

since

Managers dbudget

ISA

Binary or ternary relationship?

• Every policy belongs to an employee• Every dependent is connected to a policy• Not appropriate modeling:

agepname

DependentsCovers

name

Employees

ssn lot

Policies

policyid cost

Rather binary

• Every policy belongs to an employee• Every dependent is connected to a policy• Appropriate modeling:

Beneficiary

agepname

policyid cost

Policies

Purchaser

name

Employees

ssn lot

Dependents

Rather ternary

• Order contracts connecting Procurers (P), Suppliers (S), and Products (Pr)

• With binary relations: – P orders Pr– Supplier supplies for P– S delivers Pr

pricename

productwriter

name

Procurer

pid addr

supplier

name addr

count

Conclusions

• First step: gathering the needs• ER-model: similar to human thinking• Basic terms: entity, relationship, attribute• Other terms: weak entity, ISA relationship,

constraints

• Constraints are important• ER-model is subjective• Good ER-model gives good DB schema

• Literature:• Jeffrey D. Ullman, Jennifer Widom: A first

course in database systems• www.uni-obuda.hu/users/varkonyi.teri

Thank you for your attention!