BİL 354 Veritabanı Sistemleri. Relational Model (İlişkisel Veri Modeli)

Transcription

1 BİL 354 Veritabanı Sistemleri Relational Model (İlişkisel Veri Modeli)

2 DB Modeling & Implementation Ideas Database Model (E/R, ODL) Relational Schema Physical storage Diagrams (E/R) Tables: column names: attributes rows: tuples Complex file organization and index structures. 2

3 Example of a Relation Relation = Table attributes (or columns) tuples (or rows) 3

4 Relational terminology Relation is composed of tuples (çoklular) Tuple = sequence of attribute values Attribute has atomic types Relation schema: (İlişki Şeması) relation name + attribute names + attribute types STUDENT Database schema: set of relation schemas STUDENT INSTRUCTOR COURSE 4

5 Concepts Review Relational Model is made up of tables A row of table = a relational instance/tuple A column of table = an attribute A table = a schema/relation Cardinality = number of rows Degree = number of columns 5

6 Example of a Relation attributes (or columns) tuples (or rows) Cardinality = 12 Degree = 4 6

7 Attribute Types The set of allowed values for each attribute is called the domain(alan) of the attribute Attribute values are (normally) required to be atomic; that is, indivisible The special value null is a member of every domain The null value causes complications in the definition of many operations 7

8 Relation Schema and Instance A 1, A 2,, A n are attributes R = (A 1, A 2,, A n ) is a relation schema Example: instructor = (ID, name, dept_name, salary) Formally, given sets D 1, D 2,. D n a relation r is a subset of D 1 x D 2 x x D n Thus, a relation is a set of n-tuples (a 1, a 2,, a n ) where each a i D i The current values (relation instance (ilişki örneği)) of a relation are specified by a table An element t of r is a tuple, represented by a row in a table 8

9 Relations are Unordered Order of tuples is irrelevant (tuples may be stored in an arbitrary order) Example: instructor relation with unordered tuples 9

10 Properties of Relations Relations are sets, so the following must hold: The order of tuples is not important All the rows must be different (No duplicates) The relation should have at least one key The order of attributes is not important All the values in the attributes belong to a domain, hence have same type. Relationship values are simple, sets or structs, composite values are not allowed. 10

11 Keys Let K R K is a superkey of R if values for K are sufficient to identify a unique tuple of each possible relation r(r) Example: {ID} and {ID,name} are both superkeys of instructor. Superkey K is a candidate key if K is minimal Example: {ID} is a candidate key for Instructor One of the candidate keys is selected to be the primary key. Foreign key constraint: Value in one relation must appear in another 11

12 Reduction of an E-R Schema to Tables Primary keys allow entity sets and relationship sets 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 set and relationship set there is a unique table which is assigned the name of the corresponding entity set or relationship set. Each table has a number of columns (generally corresponding to attributes), which have unique names. 12

13 How to convert a E/R diagram to a set of relations? price name category Start Year name Product makes Company Stock price 13

14 How to translate an ER diagram to a relational schema Basic cases Entity set E => relation with attributes of E Relationship R => relation with attributes being keys of related entity sets + attributes of R 14

15 Entity Sets Entity set Students Students ssn name Rel: Students SSN Name Howard John address Address Park Avenue South Carolina 15

16 How to convert a relationship to a relation price name category Start Year name Product makes Company Makes (watch out for attribute name conflicts) Stock price Product_ Name Product_ Category Company_ Name Gizmo Gadgets GizmoWorks 1963 Start_Year 16

17 Multiway relationships & roles SSN CourseID Students enrolls Courses Name tutors graders Name TAs TA_SSN Name Different roles treated as different entity sets Key: keys of the many entities 17

18 Multiway relationships & roles Students SSN Name George Dick TAs TA_SSN Name Wesley Howard John Courses CourseID Name C Databases C Software Enrolls(S_SSN, Course_ID, Tutor_SSN, Grader_SSN) S_SSN CourseID Tutor_SSN Grader_SSN C C

19 Relationship -> Relation name addr name type husband People 1 2 Likes Sports Buddies Married wife Favorite Likes(person, sport) Favorite(person, sport) Buddies(name1, name2) Married(husband, wife) 19

20 Combining Relations OK to combine into one relation: 1. The relation for an entity-set E 2. The relations for many-one relationships of which E is the many. Example: People(name, addr) and Favorite(person, sport) combine to make Fan(name, addr, favsport) 20

21 Risk with Many-Many Relationships Combining People with Likes would be a mistake. It leads to redundancy, as: name addr sport Sally 123 Maple Basketball Sally 123 Maple Tennis Redundancy 21

22 How to convert a weak entity set to a relation Team affiliation University schedule sport name Team Sport Schedule University_Name address Basketball <long string> Hacettepe University Do include all the attributes in the key for Team. Don t include a separate relation for Affiliation. (why?) 22

23 Converting weak ESs differences Crew_ID StudioName address Crew Unit-of Studio Atts of Crew Rel are: attributes of Crew key attributes of supporting ESs Crew StudioName Miramax Miramax Disney Crew_ID C1 C2 C1 Supporting relships are omitted 23

24 Weak entity sets - relationships Crew_ID StudioName address Crew Unit-of Studio Subscribes IName Insurance IName Address Insurance Address Aetna BlueCross th Av.NY th Av.NY 24

25 Weak entity sets - relationships Non-supporting relationships for weak ESs are converted keys include entire weak ES key Subscribes StudioName Universal Universal Disney Crew_ID C21 C22 C21 Insurer Aetna BlueCross Aetna 25

26 Handling Weak Entity Sets Relation for a weak entity set must include attributes for its complete key (including those belonging to other entity sets), as well as its own, nonkey attributes. A supporting relationship is redundant and yields no relation (unless it has attributes). 26

27 Example: Weak Entity Set -> Relation name name billto Logins At Hosts location Hosts(hostName, location) Logins(loginName, hostname, billto) At(loginName, hostname, hostname2) At becomes part of Logins Must be the same 27

28 Exercise 1 Convert the E/R diagram to a relational database schema. row seat Booking Customers phone Flights SSN name address number day aircraft 28

29 How to translate a subclass memory platforms isa Product isa topic agegroup Software Product Educational Product isa isa Educ-software Product Educationalmethod 29

30 Three ways to translate subclasses Object-oriented: each entity belongs to exactly one class; create a relation for each class, with all its attributes. E/R style: create one relation for each subclass, with only the key attribute(s) and attributes attached to that entity set; Use null: create one relation; entities have nulls in attributes that don t belong to them. 30

31 Option #1: the OO Approach 4 tables: each object can only belong to a single table Product(name, price, category, manufacturer) EducationalProduct( name, price, category, manufacturer, agegroup, topic) SoftwareProduct( name, price, category, manufacturer, platforms, requiredmemory) EducationalSoftwareProduct( name, price, category, manufacturer, agegroup, topic, platforms, requiredmemory) DRAWBACK: Redundancy 31

32 Option #2: the E/R Approach Product(name, price, category, manufacturer) EducationalProduct( name, agegroup, topic) SoftwareProduct( name, platforms, requiredmemory) No need for a relation EducationalSoftwareProduct unless it has a specialized attribute: EducationalSoftwareProduct(name, educational-method) DRAWBACK: Have to reach multiple tables 32

33 Option #3: The Null Value Approach Have one table: Product ( name, price, manufacturer, age-group, topic, platforms required-memory, educational-method) Some values in the table will be NULL, meaning that the attribut not make sense for the specific product. Saves space unless there are lots of attributes that are usually NULL DRAWBACK: Redundancy and too many meanings for NULL 33

34 Representing Aggregation Name SSN Name Student Advisor Professor SID Dept Primary Key of Advisor member Dept Name SID Code Primary key of Dept Code 34