W hat is a Database Management System?

1. What is a Database Management System? Spring 2008 Lecture slides by Dr. Sara Cohen

2. What and Why • A database management system is a program used to: • Store large amounts of data • Allow easy access to the data (using a query language) • Separate the physical schema from the logical schema

3. What and Why • Protect the data from corruption or security leaks • Allow multiple users to access the data simultaneously • Provide crash recovery • And more…

4. Entity-Relationship Diagrams Spring 2008

5. Scenario • http://www.imdb.com wants to store information about movies and has chosen you to help them • Three steps: • Requirements Analysis: Discover what information needs to be stored, how the stored information will be used, etc. Taught in course on system analysis and design • Conceptual Database Design: High level description of data to be stored (ER model) • Logical Database Design: Translation of ER diagram to a relational database schema (description of tables) • Physical Database Design: Done by the DB system

6. Requirements (1) • For actors and directors, we want to store their name, a unique identification number, address and birthday (why not age?) • For actors, we also want to store a photograph • For films, we want to store the title, year of production and type (thriller, comedy, etc.) • We want to know who directed and who acted in each film. Every film has one director. We store the salary of each actor for each film

7. Requirements (2) • An actor can receive an award for his part in a film. We store information about who got which award for which film, along with the name of the award and year. • We also store the name and telephone number of the organization who gave the award. Two different organizations can give an award with the same name. A single organization does not give more than one award with a particular name per year.

8. address id birthday Movie Person name phone number name In the rest of this lesson we explain this diagram ISA Organization Gives picture Director Actor Won salary Acted In Directed Award year name Film year title type

9. ER-Diagrams: General Information • ER-diagrams are a formalism to model real-world scenarios • There are many versions of ER-diagrams that differ both in their appearance and in their meaning • We will use the version appearing in the course book (Database Management Systems by Ramakrishnan) • ER-diagrams have a formal semantics (meaning) that must be thoroughly understood, in order to create correct diagrams

10. Entities, Entity Sets • Entity (ישות): An object in the world that can be distinguished from other objects • Examples of entities: • Examples of things that are not entities: • Entity set (קבוצת ישויות): A set of similar entities • Examples of entity sets:  Entity sets are drawn as rectangles

11. Attributes • Attributes (תכונות): Used to describe entities • All entities in the set have the same attributes • A minimal set of attributes that uniquely identify an entity is called a key • An attribute contains a single piece of information (and not a list of data)

12. Attributes (2) • Examples of attributes: • Examples of things that cannot be attributes:  Attributes are drawn using ovals  The names of the attributes which make up a key are underlined

13. Example birthday id Actor name address

14. Another Option for a Key? birthday id Actor name address

15. Another Option for a Key? birthday id Actor name address

16. Relationships, Relationship Sets • Relationship (קשר): Association among two or more entities • Relationships may have attributes • Examples of Relationships: • Relationship Set (קבוצת קשרים): Set of similar relationships • Examples of Relationship sets:  Relationship sets are drawn using diamonds

17. Where does the salary attribute belong? salary Example title birthday id Film Actor year Acted In name type address

18. Recursive Relationships • An entity set can participate more than once in a relationship • In this case, we add a description of the role to the ER-diagram phone number manager id Employee Manages worker name address

19. Director id name id Actor Film Produced title name n-ary Relationship • An n-ary relationship R set involves exactly n entity sets: E1, …, En. • Each relationship in R involves exactly n entities: e1 in E1, …, enin En • Formally, R E1x …x En

20. Director id name id Actor Film Produced title name Example • Suppose that there are: • Actors: Mickey Mouse, Donald Duck • Directors: Big Bird, Kermit • Films: Mickey’s Club How many triplets can be in the relationship set “Produced”? How many pairs can be in the relationship set “Produced”?

21. Another Option: Remember Recursive Relationships

22. Important Note • The entities in a relationship set must identify the relationship • Attributes of the relationship set cannot be used for identification! • Suppose we wanted to store the role of an actor in a film. • How should we store the role of the actor? • How would we store information about a person who acted in one film in several roles? id Actor Film Acted In title name

23. Key Constraints (אילוצי מפתח) • Key constraints specify whether an entity can participate in one, or more than one, relationships in a relationship set • When there is no key constraint an entity can participate any number of times • When there is a key constraint, the entity can participate at most one time  Key constraints are drawn using an arrow from the entity set to the relationship set

24. One-to-Many • A film is directed by at most one director • A director can direct any number of films id Director Film Directed title name Director Directed Film

25. Many-to-Many • A film is directed by any number of directors • A director can direct any number of films id Director Film Directed title name Director Directed Film

26. One-to-One • A film is directed by at most one director • A director can direct at most one film id Director Film Directed title name Director Directed Film

27. Another Example Where would you put the arrow? age father id Person FatherOf child name

28. Key Constraints in Ternary Relationships Actor id name id Director Film produced title name What does this mean?

29. Participation Constraints אילוצי השתתפות)) • Participation constraints specify whether or not an entity must participate in a relationship set • When there is no participation constraint, it is possible that an entity will not participate in a relationship set • When there is a participation constraint, the entity must participate at least once  Participation constraints are drawn using a thick line from the entity set to the relationship set

30. Example (1) • A film has at lease one director • A director can direct any number of films id Director Film Directed title name Do you think that there should be a participation constraint from Director to Directed? Director Directed Film

31. Example (2) • We can combine key and participation constraints. • What does this diagram mean? id Director Film Directed title name

32. Storing Award Information • What do you think of this? • To model this correctly we need weak entity sets org_ name Won Award phone number name year

33. Weak Entity Sets • Weak entity sets are entity sets that are not uniquely identified by their attributes • A weak entity set has an "identifying relationship" with an entity set that is the "identifying owner" of the weak entity set

34. Weak Entity Sets A weak entity set must: • participate fully in the identifying relationship ( a thick line) • participate in a one to many relationship with the identifying owner (an arrow)  Weak entity sets have a thick rectangle, their keys are underlined with a broken line, and the identifying relationship has a thick diamond

35. Example phone number name Organization What would be the meaning if this was not a thick line? Gives Won Award name year

36. גדוד מספר גדוד שייכת ל פלוגה אות פלוגה שייכת ל מחלקה מספר מחלקה Example How are the entity sets identified?

37. Example • Suppose that you were storing information about books. • Should books be modeled as a weak entity set or a regular entity set? • Does ISBN identify a book • The answer: it depends what type of data you are interested in storing!

38. Copies of Books in Libraries Owned By name Library author Book title id isbn Person Copy Of Borrowed Copy copy number condition

39. ISA Hierarchies ISA Relationships: Define a hierarchy between entity sets • ISA is similar to inheritance  ISA relationships are drawn as a triangle with the word ISA inside it. The "super entity-set" is above the triangle and the "sub entity-sets" are below

40. Example • What are the keys of: • Movie Person • Actor • Director address id birthday Movie Person name ISA picture Director Actor

41. Overlap Constraints • Overlap constraints: Determine whether two sub-entity sets can contain the same entity • Example: Can an Actor be a Director?  Write "Actor OVERLAPS Director". If not written, assume no overlap

42. Covering Constraints • Covering constraints: Determine whether every entity in the super-entity set is also in at least one of the sub-entity sets • Example: Is every movie person either an Actor or a Director?  Write "Actor AND Director COVER Movie Person". If not written, assume no covering

43. Aggregation • Aggregation: Allows us to indicate that a relationship set participates in a relationship set • Remember, we want to store information about Actors, Films and their award. We will see why aggregation is needed for this…

44. What’s Wrong? All the actors for all their participation in all the films must get an award! picture Actor salary Acted In Award year Film title type

45. What’s Wrong? An actor may get an award for a film in which he has never acted in! picture Actor salary Acted In Won Award year Film title type

46. Suppose that there are: 3 actors 2 films 4 awards How many pairs can there be in ActedIn? How many pairs can there be in Won? Note that the pairs of Won are of a special type The Solution picture Actor Won salary Acted In Award year Film title type

47. address Final Diagram id birthday Movie Person name phone number name ISA Organization Gives picture Director Actor Won salary Acted In Directed Award year name Film year title type

48. References • “Database Management Systems”, by Raghu Ramakrishnan & Johannes Gehrke, third edition, McGraw-Hill, 2003. Chapters: 2

49. Assignment #2 • In the Ramakrishnan book, exercises 2.2 and 2.3 (Chapter 2). – page 52.: • Exercise 2.2: • A university DB contains information about professors (identified by social security number, or SSN) and courses (identified by courseid). Professors teach courses; each of the following situations concerns the Teachers relationship set. For each situation, draw an ER diagram that describes it (assuming no further constraints hold). • Professors can teach the same course in several semesters, and each ofering must be recorded. • Professors can teach the same course in several semesters, and only the most recent such offering needs to be recorded. (Assume this condition applies in all subsequent questions.) • Every professor must teach some course. • Every professor teaches exactly one course. • Every professor teaches exactly one course, and every course must be taught by some professor. • Now suppose that certain courses can be taught by a team of professors jointly, but it is possible that no one professor in a team can teach the course.

50. Assignment #2 • Exercise 2.3: • Design and draw ER diagram capturing all the following constraints regarding an university DB: • Professors have an SSN, a name, an age, a rank, and a research specialty. • Projects have a project number, a sponsor name, a starting date, an ending date, and a budget. • Graduate students have an SSN, a name, an age, and a degree program (M.S. or Ph.D.) • Each project is managed by one professor (known as the project’s principal investigator). • Each project is worked on by one or more professors (known as the project’s co-investigators). • Professors can manage and/or work on multiple projects. • Each project is worked on by one or more graduate students (the project’s research assistants).