1. Lecture 3 Entity Relationship diagram (ERD) Cardinalities
Converting ERD into tables (relations)
An ERD example
Q8/p139
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2. Entities An object of interest Person Place Event Concept Ex: Student
Order
Customer
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3. Super & Sub Type Entities
Recognize common theme
WORKER (SUPER TYPE)
PART TIME (SUB TYPE)
FULL TIME (SUB TYPE)
CONTRACTUAL (SUB TYPE)
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4. Relationship “Has A” between entities
“IS A” between super & sub entities
TEAM
PLAYER
WORKER
PART TIME
FULL TIME
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5. INHERITANCE
Sub type entities inherit all attributes & procedures of its SUPER entity type
Also the basis of Objects
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6. Attributes Properties of an entity Describes an entity
Ex: attribute of an entity STUDENT
student name
student address
etc
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7. Domain Set of allowable values for an attribute Ex:
Student_age (between 15-85)
Student _classification
(Graduate or
Undergraduate)
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8. Relationship Exists between entities Recursive ..within itself
Binary between TWO entities
Ternary between THREE entities
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9. Nature of Relationship between ENTITIES
1:1 (one is to one)
1:m (one to many)
m:n (many to many)
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10. Recursive
An entity has relationship to itself, it could be 1:1; 1:m or m:n
EX: EMPLOYEES MARRIED TO OTHER EMPLOYEES
EMPLOYEE
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11. CREATE A TABLE WITH EMP_SPOUSE NUMBER ADDED TO IT
EMPLOYEE
(EMP_NO, EMP_NAME,…,SPOUSE_NO)
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12. Cardinalities
Relates specific occurrences of ONE entity to other, and vice-versa
MIN cardinality
MAX cardinality
STUDENT
COURSE
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13. Min, Max STUDENT <------->CLASS
What’s the minimum number of classes a student can take zero, what’s the max, say 6
What’s the minimum number of students a class can have say 1, max say 40 (class limit), then
STUDENT< >CLASS
(0,6) (1,40)
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14. Another Cardinality Example
Patient Pat_history
What’s the minimum number of patient history a patient can have? 1
What’s the maximum ? Many, say N
What’s the minimum number of patient a patient history can belong to? Maximum?
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15. Patient Pat_history
(1, N) (1,1)
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16. Converting 1:1; 1:m and m:n into relations
Each entity becomes a relation and
PK of either entity becomes a FK in the other entity
ASSUME AN EMPLOYEE CAN have only one car and a car belongs to only ONE employee
EMPLOYEE CAR
Relations:
EMPLOYEE (emp_no, emp_name,..,car_id)
CAR (car_id, model, make…)
Note: or emp_no could be FK in CAR
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17. 1:m PK of PARENT becomes FK in the CHILD Ex:
Assume: A team can have many players and a player can play for ONLY ONE team
TEAM PLAYER
TEAM (Team_name, location, manager,..)
PLAYER (Player_name,…Team_Name)
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18. M:N Convert m:n into TWO 1:m by introducing an intersection entity
The intersection contains PK of both entities
CLASS STUDENT
CLASS GRADE STUDENT
CLASS (class_id, # of students,..)
STUDENT (st_id, st_name, GPA,..)
GRADE (class_id, st_id, semester, grade,..)
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19. EX: Convert following ERD into relations
assume a student can be advised by one professor but a professor can advise many students, a club can have many students but a student can join only one club, then the ERD will look as
STUDENT
PROFESSOR CLUB
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20. Relations will be STUDENT
(St_ID, st_name,…,Club_ID, Prof_ID)
Note: we must account for both relationships in STUDENT entity (student, professor) AND (student, club)
CLUB
(Club_ID, location,date_established,..)
PROFESSOR
(Prof_ID, Prof_name, specialty,..)
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21. Building an ER model Q8/P139
Identify entities
DINNER
ENTRÉE
GUEST
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22. Relationships
Each dinner is based on a single entrée, but each entrée can be served at many dinners
DINNER
ENTREE
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23. Cardinality Cardinality
One entrée can be part of a min of 1 dinner and a max of many dinners, say n
One dinner can be part of a minimum of one entrée and a max of one entrée
(1,N) (1,1)
entree
dinner
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24. Relationship between DINNER and GUEST
Note there are TWO relationships
A guest can attend many dinners
Vice versa
A guest can have many dinner invitations
invitation
Dinner
Guest
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25. Relationship between DINNER and GUEST
Note there are TWO relationships
A guest can attend many dinners
Vice versa
Dinner
Guest
Attend
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26. Relationship between DINNER and GUEST
Second relationship
A guest can have many dinner invitations
Vice versa
invitation
Dinner
Guest
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27. Converting m:n into two 1:m
since Guest and DINNER is m:n we can break into TWO 1:m
i.e ATTEND entity is intersection of DINNER & GUEST
DINNER ATTEND GUEST
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28. cardinality
What is ATTEND entity ? a combination of a dinner and a guest
What’s the min number of DINNER in ATTEND? 1
(a dinner must be part of at lest ONE function (ATTEND)
Max number? N (assuming N is total # of dinners)
An attend can be part as a min part of one dinner and a max of ONE dinner also
what’s the min number of ATTEND a GUEST can have?0
(a guest may not attend any dinner) or they may attend a max of N dinners
DINNER ATTEND GUEST
(1,N) (1,1) (1,1) (0,N)
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29. Same concepts can be applied to INVITATION intersection entity
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30. ERD Q 8/p139
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