1. Faloutsos - Pavlo C. Faloutsos - A. Pavlo Lecture#2: E-R diagrams
CMU SCS /615
Carnegie Mellon Univ. Dept. of Computer Science /615 - DB Applications
C. Faloutsos - A. Pavlo
Lecture#2: E-R diagrams

2. Problem Develop an application for U.G. admin: How do you proceed?
Faloutsos - Pavlo
CMU SCS /615
Problem
Develop an application for U.G. admin:
Student info
Who-takes-what class
Class rosters
Transcripts
How do you proceed?
(Which role(s) are you playing?)
Faloutsos - Pavlo
CMU SCS /615

3. Database Design Requirements Analysis Conceptual Design Logical Design
Faloutsos - Pavlo
CMU SCS /615
Database Design
Requirements Analysis
Conceptual Design
Logical Design
Schema Refinement
Physical Design
Security Design
Faloutsos - Pavlo
CMU SCS /615

4. Database Design Requirements Analysis Conceptual Design Logical Design
Faloutsos - Pavlo
CMU SCS /615
Database Design ✔
Requirements Analysis
Conceptual Design
Logical Design
Schema Refinement
Physical Design
Security Design
user’s needs
high level (ER)
Tables
Normalization
Indices etc
Access controls
Faloutsos - Pavlo
CMU SCS /615

5. Problem’ Maintain Develop an application for U.G. admin:
Faloutsos - Pavlo
CMU SCS /615
Problem’
Maintain
Develop an application for U.G. admin:
Student info
Who-takes-what class
Class rosters
Transcripts
If you are the *new* DBA, what would you rather inherit:
Faloutsos - Pavlo
CMU SCS /615

6. This or this ? Student Takes Course drop table if exists student;
create table student
(ssn fixed,
name char(20));
drop table if exists takes;
create table takes
cid char(10),
grade fixed);
Student
Course
Takes
Faloutsos - Pavlo
CMU SCS /615

7. True story Health insurance company
Wants to catch (some of the abundant) fraud
Schema:
patients, visit doctors, get medicine,
Doctors perform operations, …
Nurses monitor patients, …
etc etc
Q: How many tables do you think it spans?
Faloutsos - Pavlo
CMU SCS /615

8. True story
Schema:
patients, visit doctors, get medicine,
Doctors perform operations, …
Nurses monitor patients, …
etc etc
Q: How many tables do you think it spans? 10? 20? 30?
Faloutsos - Pavlo
CMU SCS /615

9. True story
Schema:
patients, visit doctors, get medicine,
Doctors perform operations, …
Nurses monitor patients, …
etc etc
Q: How many tables do you think it spans? 10? 20? 30?
A: 120 PAGES of schema
Faloutsos - Pavlo
CMU SCS /615

10. Motivation & upcoming conclusion:
E-R diagrams are excellent documentation tools
Student
Course
Takes
Faloutsos - Pavlo
CMU SCS /615

11. Overview concepts Entities Relationships Attributes
Specialization/Generalization
Aggregation
ER modeling questions
Faloutsos - Pavlo
CMU SCS /615

12. Tools Entities (‘entity sets’) Relationships (‘rel. sets’)
Student
Course
Takes
Tools
Entities (‘entity sets’)
Relationships (‘rel. sets’)
and mapping constraints N M P
attributes
Faloutsos - Pavlo
CMU SCS /615

13. Example
Students, taking courses, offered by instructors; a course may have multiple sections; one instructor per section
nouns -> entity sets
verbs -> relationship sets
Faloutsos - Pavlo
CMU SCS /615

14. Example
Students, taking courses, offered by instructors; a course may have multiple sections; one instructor per section
nouns -> entity sets
verbs -> relationship sets
Faloutsos - Pavlo
CMU SCS /615

15. Example
Students, taking courses, offered by instructors; a course may have multiple sections; one instructor per section
nouns -> entity sets
verbs -> relationship sets
Faloutsos - Pavlo
CMU SCS /615

16. unique identifier ->
...
name
STUDENT
ssn
INSTRUCTOR
issn
primary key =
unique identifier ->
underline
Faloutsos - Pavlo
CMU SCS /615

17. but: sections of course (with different instructors)?
...
name
STUDENT
c-id
ssn
c-name
COURSE
INSTRUCTOR
issn
but: sections of course (with different instructors)?
Faloutsos - Pavlo
CMU SCS /615

18. but: s-id is not unique... (see later) INSTRUCTOR issn
STUDENT
c-id
COURSE
SECTION
s-id
but: s-id is not unique... (see later)
INSTRUCTOR
issn
Faloutsos - Pavlo
CMU SCS /615

19. Q: how to record that students take courses? INSTRUCTOR issn
c-id
COURSE
SECTION
s-id
Q: how to record that students take courses?
INSTRUCTOR
issn
Faloutsos - Pavlo
CMU SCS /615

20. ssn STUDENT N c-id takes M COURSE s-id SECTION INSTRUCTOR issn
Faloutsos - Pavlo
CMU SCS /615

21. STUDENT N c-id takes M s-id SECTION COURSE N teaches 1 INSTRUCTOR
Faloutsos - Pavlo
CMU SCS /615

22. STUDENT N c-id takes M N 1 s-id has SECTION COURSE N teaches 1
INSTRUCTOR
Faloutsos - Pavlo
CMU SCS /615

23. Cardinalities 1 to 1 (example?) 1 to N N to M 1 1 1 N N M
Faloutsos - Pavlo
CMU SCS /615

24. Cardinalities 1 1 has CAPITAL 1 N owns PERSON CAR N M takes SECTION
COUNTRY
CAPITAL 1 N
owns
PERSON
CAR N M
takes
SECTION
STUDENT
Faloutsos - Pavlo
CMU SCS /615

25. Cardinalities Book’s notation: has CAPITAL owns PERSON CAR takes
COUNTRY
CAPITAL
Book’s notation:
owns
PERSON
CAR
takes
SECTION
STUDENT
Faloutsos - Pavlo
CMU SCS /615

26. Cardinalities Book’s notation vs 1 to N notation 1 1 has CAPITAL 1 N
COUNTRY
CAPITAL
Book’s notation vs
1 to N notation 1 N
owns
PERSON
CAR N M
takes
SECTION
STUDENT
Faloutsos - Pavlo
CMU SCS /615

27. Cardinalities 1 1 has CAPITAL 1 N owns PERSON CAR N M takes SECTION
COUNTRY
CAPITAL 1 N
owns
PERSON
CAR N M
takes
SECTION
STUDENT
Faloutsos - Pavlo
CMU SCS /615

28. ‘Total/partial’ participation
1:1
1:1
total, total
has
COUNTRY
CAPITAL
?:1
?:N
owns
PERSON
CAR ??
?:N
?:M
takes
SECTION ??
STUDENT
Faloutsos - Pavlo
CMU SCS /615

29. ‘Total/partial’ participation
1:1
1:1
total, total
has
COUNTRY
CAPITAL
1:1
0:N
owns
PERSON
CAR
partial, total
?:N
?:M
takes
SECTION ??
STUDENT
Faloutsos - Pavlo
CMU SCS /615

30. ‘Total/partial’ participation
1:1
1:1
total, total
has
COUNTRY
CAPITAL
1:1
0:N
owns
PERSON
CAR
partial, total
1:N
0:M
takes
SECTION
partial, total
STUDENT
Faloutsos - Pavlo
CMU SCS /615

31. ‘Total/partial’ participation
Is it ‘legal’?
partial, total
1:1
0:N
owns
PERSON
CAR
Faloutsos - Pavlo
CMU SCS /615

32. ‘Total/partial’ participation
Is it ‘legal’?
partial, total
NO! why not?
1:1
0:N
owns
PERSON
CAR
Faloutsos - Pavlo
CMU SCS /615

33. Subtle concept: Weak entities
‘section’ has no unique-id of its own!(?)
c-id N 1
s-id
has
SECTION
COURSE
Faloutsos - Pavlo
CMU SCS /615

34. Weak entities
‘weak’ entities: if they need to borrow a unique id from a ‘strong entity - thick box.
‘c-id’ + ‘s-id’: unique id for SECTION
partial key (eg., ‘s-id’) - dashed-underline
identifying relationship (eg., ‘has’)
c-id N 1
s-id
SECTION
has
COURSE
Faloutsos - Pavlo
CMU SCS /615

35. Weak entities Other example(s) of weak entities? ??id N 1 ?? ??? ??
Faloutsos - Pavlo
CMU SCS /615

36. Weak entities Other example(s) of weak entities? e-id N 1 name
Dependent
has
Employee
Faloutsos - Pavlo
CMU SCS /615

37. More details self-relationships - example? ?? ?? ?? ??
Faloutsos - Pavlo
CMU SCS /615

38. More details self-relationships - example? manages 1 EMPLOYEE N
Faloutsos - Pavlo
CMU SCS /615

39. More details self-relationships - example? Has-friend ?? FB user ??
Faloutsos - Pavlo
CMU SCS /615

40. More details 3-way and k-way relationships? Faloutsos - Pavlo
CMU SCS /615

41. More details 3-way and k-way relationships? Rare, but possible: N M
EMPLOYEE
TOOL
uses P
PROJECT
Faloutsos - Pavlo
CMU SCS /615

42. More details 3-way and k-way relationships? Rare, but possible: N M ??
Other cases? ??
Faloutsos - Pavlo
CMU SCS /615

43. More details 3-way and k-way relationships? Rare, but possible: N M
reviews
user
keyword P
App-store/amazon
reviews
app
Faloutsos - Pavlo
CMU SCS /615

44. Overview concepts Entities Relationships Attributes
Specialization/Generalization
Aggregation
ER modeling questions
Faloutsos - Pavlo
CMU SCS /615

45. More details - attributes
key (or primary key): unique identifier
underlined, in the ER diagram
[not in textbook - FYI:
multivalued or set-valued attributes (eg., ‘dependents’ for EMPLOYEE)
derived attributes (eg., 15% tip) ]
Faloutsos - Pavlo
CMU SCS /615

46. Overview concepts Basic Advanced/ rare Entities Relationships
Attributes
Specialization/Generalization
Aggregation
ER modeling questions
Basic
Advanced/
rare
Faloutsos - Pavlo
CMU SCS /615

47. Specialization
eg., students: part time (#credit-hours) and full time (major)
name
STUDENT
ssn
IS-A
FT-STUDENT
PT-STUDENT
major
#credits
Faloutsos - Pavlo
CMU SCS /615

48. Observations Generalization: exact reverse of ‘specialization’
attribute inheritance
could have many levels of an IS-A hierarchy
Faloutsos - Pavlo
CMU SCS /615

49. More details Overlap constraints Covering constraints A B C
Faloutsos - Pavlo
CMU SCS /615

50. More details Overlap constraints Covering constraints
can an entity belong to both ‘B’ and ‘C’?
Covering constraints
can an ‘A’ entity belong to neither ‘B’ nor ‘C’? B C
Faloutsos - Pavlo
CMU SCS /615

51. More details Overlap constraints - examples? A No overlap with overlapB C A A C C B B
Faloutsos - Pavlo
CMU SCS /615

52. More details Covering constraints - examples? A Total coverage A C B B
Partial coverage A C B
Faloutsos - Pavlo
CMU SCS /615

53. Overview concepts Entities Relationships Attributes
Specialization/Generalization
Aggregation
ER modeling questions
Faloutsos - Pavlo
CMU SCS /615

54. Aggregation computer model (w/ CPU and HD) and Maker (eg., Dell, HP) M?
MAKER HD
CPU
Faloutsos - Pavlo
CMU SCS /615

55. Aggregation treat a relationship as an entity
used to express a relationship among relationships
CPU HD N M
MAKER
Faloutsos - Pavlo
CMU SCS /615

56. Overview concepts Entities Relationships Attributes
Specialization/Generalization
Aggregation
ER modeling questions
Faloutsos - Pavlo
CMU SCS /615

57. Conceptual design Entity vs attribute Entity vs relationship
Binary or ternary relationships?
Aggregation?
Faloutsos - Pavlo
CMU SCS /615

58. Entity vs. attribute Entity EMPLOYEE (w/ emp#, name, job_code, ...)
Q: How about ‘spouse’ - entity or attribute?
Q: How about ‘dependents’?
Faloutsos - Pavlo
CMU SCS /615

59. Entity vs. attribute Entity EMPLOYEE (w/ emp#, name, job_code, ...)
Q: How about ‘spouse’ - entity or attribute?
A: probably, ‘attribute’ is enough
Q: How about ‘dependents’?
A: Entity - we may have many dependents
Faloutsos - Pavlo
CMU SCS /615

60. Entity vs. Relationship
STUDENT
STUDENT 1 N N OR
takes
TAKES M N
SECTION 1
SECTION
Faloutsos - Pavlo
CMU SCS /615

61. Binary vs Ternary Relationships
usually, binary relationships are ‘cleaner’:
Faloutsos - Pavlo
CMU SCS /615

62. Binary vs. Ternary Relationships
Faloutsos - Pavlo
CMU SCS /615
age
pname
Dependents
Covers
name
Employees
ssn
lot
If each policy is owned by just 1 employee:
Policies
policyid
cost
Faloutsos - Pavlo
CMU SCS /615 7

63. Binary vs. Ternary Relationships
Faloutsos - Pavlo
CMU SCS /615
Policies
policyid
cost
age
pname
Dependents
Covers
name
Employees
ssn
lot
If each policy is owned by just 1 employee:
Bad design
Faloutsos - Pavlo
CMU SCS /615 7

64. Binary vs. Ternary Relationships
Faloutsos - Pavlo
CMU SCS /615
Policies
policyid
cost
age
pname
Dependents
Covers
name
Employees
ssn
lot
If each policy is owned by just 1 employee:
Bad design
Key constraint on Policies would mean policy can only cover 1 dependent!
Faloutsos - Pavlo
CMU SCS /615 7

65. Binary vs. Ternary Relationships
Faloutsos - Pavlo
CMU SCS /615
Policies
policyid
cost
age
pname
Dependents
Covers
name
Employees
ssn
lot
If each policy is owned by just 1 employee:
Bad design
Key constraint on Policies would mean policy can only cover 1 dependent!
Beneficiary
age
pname
Dependents
policyid
cost
Policies
Purchaser
name
Employees
ssn
lot
Better design
What are the additional constraints in the 2nd diagram?
Faloutsos - Pavlo
CMU SCS /615 7

66. Binary vs Ternary Rel.
But sometimes ternary rel. can not be replaced by a set of binary rel’s:
Faloutsos - Pavlo
CMU SCS /615

67. Binary vs. Ternary Relationships (Contd.)
Faloutsos - Pavlo
CMU SCS /615
Binary vs. Ternary Relationships (Contd.)
qty
Parts
Contract
Departments
VS.
Suppliers
Parts
Departments
can-supply
Suppliers
deals-with
why is it bad?
Faloutsos - Pavlo
CMU SCS /615 9

68. Binary vs. Ternary Relationships (Contd.)
Faloutsos - Pavlo
CMU SCS /615
Binary vs. Ternary Relationships (Contd.)
qty
Parts
Contract
Departments
VS.
Suppliers
Parts
Departments
can-supply
Suppliers
deals-with
S “can-supply” P, D “needs” P, and D “deals-with” S does not imply that D has agreed to buy P from S.
How do we record qty?
Faloutsos - Pavlo
CMU SCS /615 9

69. Binary vs. Ternary Relationships (Contd.)
Faloutsos - Pavlo
CMU SCS /615
Binary vs. Ternary Relationships (Contd.)
qty
Not in textbook:
in practice, often:
Parts
Contract
Departments
Suppliers
qty
Parts
Contract
Departments
Suppliers
Faloutsos - Pavlo
CMU SCS /615 9

70. Binary vs. Ternary Relationships (Contd.)
Faloutsos - Pavlo
CMU SCS /615
Binary vs. Ternary Relationships (Contd.)
Not in textbook:
in practice, often:
c-id
qty
Parts
Contract
Departments
Suppliers
Faloutsos - Pavlo
CMU SCS /615 9

71. Binary vs. Ternary Relationships (Contd.)
Faloutsos - Pavlo
CMU SCS /615
Binary vs. Ternary Relationships (Contd.)
Not in textbook:
in practice, often:
c-id
qty M N
Parts
Contract
Departments N N 1 1
Suppliers
Faloutsos - Pavlo
CMU SCS /615 9

72. Ternary vs. aggregation
use aggregation, if we want to attach a relationship to a relationship
(see book for example)
(in practice, again we create a unique-id and resort to binary relationships)
Faloutsos - Pavlo
CMU SCS /615

73. Ternary vs. aggregation
How would you handle this case?
CPU HD N M
MAKER
Faloutsos - Pavlo
CMU SCS /615

74. Ternary vs. aggregation
How would you handle this case?
COMP. MODEL
MAKER HD
CPU
Faloutsos - Pavlo
CMU SCS /615

75. Ternary vs. aggregation
How would you handle this case?
COMP. MODEL ? ? ? ?
MAKER HD
CPU
Faloutsos - Pavlo
CMU SCS /615

76. Ternary vs. aggregation
How would you handle this case?
COMP. MODEL M M
HAS_CPU N N
MAKER HD
CPU
Faloutsos - Pavlo
CMU SCS /615

77. Summary
E-R Diagrams: a powerful, user-friendly tool for data modeling:
Entities (strong, weak)
Attributes (primary keys, discriminators, derived, multivalued)
Relationships (1:1, 1:N, N:M; multi-way)
Generalization/Specialization; Aggregation
Faloutsos - Pavlo
CMU SCS /615

78. Summary
E-R Diagrams: a powerful, user-friendly tool for data modeling:
Entities (strong, weak)
Attributes (primary keys, discriminators, derived, multivalued)
Relationships (1:1, 1:N, N:M; multi-way)
Generalization/Specialization; Aggregation
POPULAR
Faloutsos - Pavlo
CMU SCS /615

79. Summary - cont’d attribute (strong) entity set weak entity set
primary key A
relationship set
partial key A
identifying rel. set
for weak entity
Faloutsos - Pavlo
CMU SCS /615

80. Summary - cont’d M N cardinalities cardinalities l’:h’ l:h
with limits
l:h
l’:h’
partial/total
(not in textbook - FYI)
Faloutsos - Pavlo
CMU SCS /615

81. Summary - cont’d
IS-A
aggregation
Faloutsos - Pavlo
CMU SCS /615