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

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

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 Actor id name address birthday

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

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

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

18 Recursive Relationships Employee id name address phone number Manages 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 manager worker

19 n-ary Relationship An n-ary relationship R set involves exactly n entity sets: E 1, …, E n. Each relationship in R involves exactly n entities: e 1 in E 1, …, e n in E n Formally, R  E 1 x …x E n Actor id name Produced Film title Director idname

20 Example Suppose that there are: –Actors: Mickey Mouse, Donald Duck –Directors: Big Bird, Kermit –Films: Mickey’s Club Actor id name Produced Film title Director idname 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 Actor id name Acted In Film title 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?

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 Director id name Directed Film title DirectorDirectedFilm

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

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

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

28 Key Constraints in Ternary Relationships Director id name produced Film title Actor idname 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 Director id name Directed Film title DirectorDirectedFilm Do you think that there should be a participation constraint from Director to Directed?

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

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

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

36 Example פלוגה שייכת ל אות פלוגה מחלקה מספר מחלקה גדוד מספר גדוד שייכת ל 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 Book title author isbn Copy copy numbercondition Copy Of Borrowed Person id Owned By Library name

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: 1.Movie Person 2.Actor 3.Director Movie Person ISA Actor id name address birthday picture Director

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

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

46 The Solution Actor picture Film year type title Acted In salary Award Won 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

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

48 References “Database Management Systems”, by Raghu Ramakrishnan & Johannes Gehrke, third edition, McGraw-Hill, 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).

51 Assignment #2 Exercise 2.3 – cont.: When grad. students work on a project, a professor must supervise their work on the project. Grad. Students may work on many projects (in this case they may have more than one supervisor). Departments have a department number, name, and a main office. Departments have a professor, who runs the department. Professors work in one or more departments, and for each department that they work in, a time percentage is associated with their job. Grad. students have one major department in which they are working on their degree. Each grad. Student has another, more senior grad. student (a student advisor) who advises him/her on what courses to take.