Rensselaer Polytechnic Institute CSCI-4380 – Database Systems David Goldschmidt, Ph.D.

 Modeling a database begins by identifying the information to be stored  Need to also define how information elements are related to one another  Further, define constraints on the information, including defining keys, referential integrity, etc.

 An entity is a building block of our database, abstracting an object of some sort  e.g. movie, faculty member, student  An entity set is a collection of similar entities  e.g. all movies, all faculty members, all students  This is similar to the object/class concepts of OOP, but there are no methods/functions

 Each entity set has a key and (usually) other attributes  The key consists of one or more attributes  The key should functionally determine all other attributes!  Entity sets should be in BCNF or 3NF  Entity sets should not have attributes that relate to other entity sets, unless they are foreign keys

 An E/R diagram is a graph representing:  Entity sets (rectangles)  Attributes (ovals)  Relationships (diamonds) ▪ i.e. connections between two or more entity sets Students Courses id name enroll-in E/R diagrams are a notation for describing database schemas keys are underlined

 Each course has many enrolled students  Each students enrolls in many classes  Note that many implies zero or more Students Courses enroll-in

 Each department has many faculty  Each faculty member belongs to at most one department (so zero or one) Faculty Departments in “at most one”

 Each department has at most one chair  Each faculty member can be chair of at most one department Faculty Departments chair-of

 For a particular major and student, there is at most one faculty member who is the advisor  A faculty member may advise many students in many majors Students Faculty advised-by Majors

 A student enrolls in a course with at most one department code (e.g. CSCI)  Is this the correct relationship? Students Courses enroll-in DeptCodes (e.g. CSCI)

 The department code is actually functionally determined by the course itself (e.g. by CRN)  Note that this does not account for cross-listed courses.... Students Courses enroll-in DeptCodes (e.g. CSCI) has

 Is this the correct relationship?  It is correct (and necessary) if a faculty member can advise students across multiple majors  Otherwise, each faculty should functionally determine the major s/he advises Students Faculty advised-by Majors

 A movie may have many sequels  For each sequel, there is at most one original movie Movies sequel-of original sequel exactly one in this case... role

 The grade attribute is associated with the given relationship ▪ And involves both a student and a course  This is merely a shortcut.... Students Courses enroll-in grade

 The grade attribute of the Grades entity set (relation) is functionally determined by the entire tuple involving Students and Courses  i.e. a student enrolled in a course has a grade Students Courses enroll-in Grades grade

 Go back to the relations we’ve worked with in class and create E/R diagrams for each  e.g. MusicGroup, Company  Also create a detailed E/R diagram for the celebrities relations (and relationships!)  see next slide....