1. CPSC-310 Database Systems
Professor Jianer Chen
Room 315C HRBB
Lecture #15

2. Outline of Course Representing things by tables E-R model (Ch. 4)
Good table structures
Functional Dependencies (Ch.3)
Basic operations on relations
Relational Algebra (Chs. 2+5)
Storage management
(Chs )
SQL languages in DDL/DML
(Ch. 6)
Query processing
(Chs )
More on SQL
(Chs. 7-9)
Transition processing
(Chs )

3. Outline of Course Representing things by tables E-R model (Ch. 4)
Good table structures
Functional Dependencies (Ch.3)
Basic operations on relations
Relational Algebra (Chs. 2+5)
Storage management
(Chs )
SQL languages in DDL/DML
(Ch. 6)
Query processing
(Chs )
More on SQL
(Chs. 7-9)
Transition processing
(Chs )

4. Our Running Example
Our SQL queries will be based on the following database schema (Underline indicates key attributes)
Beers(name, manf)
Bars(name, addr, license)
Drinkers(name, addr, phone)
Likes(drinker, beer)
Sells(bar, beer, price)
Frequents(drinker, bar)

5. Constraints and Triggers

6. Constraints and Triggers
A constraint is a relationship among data elements that the DBMS is required to enforce.

7. Constraints and Triggers
A constraint is a relationship among data elements that the DBMS is required to enforce.
-- e.g., key constraints.

8. Constraints and Triggers
A constraint is a relationship among data elements that the DBMS is required to enforce.
-- e.g., key constraints.
A trigger is an action only executed when a specified condition occurs, e.g., insertion of a tuple.

9. Constraints and Triggers
A constraint is a relationship among data elements that the DBMS is required to enforce.
-- e.g., key constraints.
A trigger is an action only executed when a specified condition occurs, e.g., insertion of a tuple.
-- easier to implement than complex constraints.

10. Constraints and Triggers
A constraint is a relationship among data elements that the DBMS is required to enforce.
-- e.g., key constraints.
A trigger is an action only executed when a specified condition occurs, e.g., insertion of a tuple.
-- easier to implement than complex constraints.

11. Kinds of Constraints

12. Kinds of Constraints
Keys (unique, cannot be NULL).

13. Kinds of Constraints Keys (unique, cannot be NULL). Foreign-key
-- referential-integrity.

14. Kinds of Constraints Keys (unique, cannot be NULL). Foreign-key
-- referential-integrity.
Value-based constraints.
-- constrain values of a particular attribute.

15. Kinds of Constraints Keys (unique, cannot be NULL). Foreign-key
-- referential-integrity.
Value-based constraints.
-- constrain values of a particular attribute.
Tuple-based constraints.
-- relationship among components.

16. Kinds of Constraints Keys (unique, cannot be NULL). Foreign-key
-- referential-integrity.
Value-based constraints.
-- constrain values of a particular attribute.
Tuple-based constraints.
-- relationship among components.
Assertions.
-- any SQL boolean expression.

17. Foreign Keys Consider the relation Sells(bar, beer, price).
Beers(name, manf)
Foreign Keys
Consider the relation Sells(bar, beer, price).
We might expect that a beer value in Sells is a real beer --- something appearing in Beers.name.

18. Foreign Keys Consider the relation Sells(bar, beer, price).
Beers(name, manf)
Foreign Keys
Consider the relation Sells(bar, beer, price).
We might expect that a beer value in Sells is a real beer --- something appearing in Beers.name.
A constraint that requires a beer in Sells to be a beer in Beers is called a foreign-key constraint.

19. Expressing Foreign Keys

20. Expressing Foreign Keys
Use the keyword REFERENCES, either:

21. Expressing Foreign Keys
Use the keyword REFERENCES, either:
-- Within the declaration of an attribute (only for one-attribute keys).

22. Expressing Foreign Keys
Use the keyword REFERENCES, either:
-- Within the declaration of an attribute (only for one-attribute keys).
-- As an element of the schema:
FOREIGN KEY (<list of attributes>)
REFERENCES <relation> (<attributes>)

23. Expressing Foreign Keys
Use the keyword REFERENCES, either:
-- Within the declaration of an attribute (only for one-attribute keys).
-- As an element of the schema:
FOREIGN KEY (<list of attributes>)
REFERENCES <relation> (<attributes>)
Referenced attributes must be declared PRIMARY KEY or UNIQUE in <relation>.

24. Example: With Attribute
Sells(bar, beer, price)
Beers(name, manf)
Example: With Attribute
CREATE TABLE Beers (
name CHAR(20) PRIMARY KEY,
manf CHAR(20) );

25. Example: With Attribute
Sells(bar, beer, price)
Beers(name, manf)
Example: With Attribute
CREATE TABLE Beers (
name CHAR(20) PRIMARY KEY,
manf CHAR(20) );
CREATE TABLE Sells (
bar CHAR(20),
beer CHAR(20)
REFERENCES Beers(name),
price REAL );

26. Example: With Attribute
Sells(bar, beer, price)
Beers(name, manf)
Example: With Attribute
CREATE TABLE Beers (
name CHAR(20) PRIMARY KEY,
manf CHAR(20) );
CREATE TABLE Sells (
bar CHAR(20),
beer CHAR(20)
REFERENCES Beers(name),
price REAL );

27. Example: As Element CREATE TABLE Beers ( name CHAR(20) PRIMARY KEY,
Sells(bar, beer, price)
Beers(name, manf)
Example: As Element
CREATE TABLE Beers (
name CHAR(20) PRIMARY KEY,
manf CHAR(20) );

28. Example: As Element CREATE TABLE Beers ( name CHAR(20) PRIMARY KEY,
Sells(bar, beer, price)
Beers(name, manf)
Example: As Element
CREATE TABLE Beers (
name CHAR(20) PRIMARY KEY,
manf CHAR(20) );
CREATE TABLE Sells (
bar CHAR(20),
price REAL,
FOREIGN KEY(beer)
REFERENCES Beers(name));

29. Example: As Element CREATE TABLE Beers ( name CHAR(20) PRIMARY KEY,
Sells(bar, beer, price)
Beers(name, manf)
Example: As Element
CREATE TABLE Beers (
name CHAR(20) PRIMARY KEY,
manf CHAR(20) );
CREATE TABLE Sells (
bar CHAR(20),
price REAL,
FOREIGN KEY(beer)
REFERENCES Beers(name));

30. Example: As Element CREATE TABLE Beers ( name CHAR(20) PRIMARY KEY,
Sells(bar, beer, price)
Beers(name, manf)
Example: As Element
CREATE TABLE Beers (
name CHAR(20) PRIMARY KEY,
manf CHAR(20) );
CREATE TABLE Sells (
bar CHAR(20),
price REAL,
FOREIGN KEY(beer)
REFERENCES Beers(name));
can be a list
of more than
one attributes

31. Example: As Element CREATE TABLE Beers ( name CHAR(20) PRIMARY KEY,
Sells(bar, beer, price)
Beers(name, manf)
Example: As Element
CREATE TABLE Beers (
name CHAR(20) PRIMARY KEY,
manf CHAR(20) );
CREATE TABLE Sells (
bar CHAR(20),
price REAL,
FOREIGN KEY(beer)
REFERENCES Beers(name));
Remark.
Attributes in
a foreign key
MAY have
value NULL
can be a list
of more than
one attributes

32. Enforcing Foreign-Key Constraints
If there is a foreign-key constraint from attributes of relation R to a key of relation S, two violations are possible:

33. Enforcing Foreign-Key Constraints
If there is a foreign-key constraint from attributes of relation R to a key of relation S, two violations are possible:
-- An insert or update to R introduces values not found in S.

34. Enforcing Foreign-Key Constraints
Sells(bar, beer, price)
Beers(name, manf)
Enforcing Foreign-Key Constraints
If there is a foreign-key constraint from attributes of relation R to a key of relation S, two violations are possible:
-- An insert or update to R introduces values not found in S.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
name
manf
Bud
A.B.
Miller
MBC

35. Enforcing Foreign-Key Constraints
Sells(bar, beer, price)
Beers(name, manf)
Enforcing Foreign-Key Constraints
If there is a foreign-key constraint from attributes of relation R to a key of relation S, two violations are possible:
-- An insert or update to R introduces values not found in S.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
Bud Lite
3.20
name
manf
Bud
A.B.
Miller
MBC

36. Enforcing Foreign-Key Constraints
Sells(bar, beer, price)
Beers(name, manf)
Enforcing Foreign-Key Constraints
If there is a foreign-key constraint from attributes of relation R to a key of relation S, two violations are possible:
-- An insert or update to R introduces values not found in S.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
Bud Lite
3.20
name
manf
Bud
A.B.
Miller
MBC
No Bud Lite

37. Enforcing Foreign-Key Constraints
Sells(bar, beer, price)
Beers(name, manf)
Enforcing Foreign-Key Constraints
If there is a foreign-key constraint from attributes of relation R to a key of relation S, two violations are possible:
-- An insert or update to R introduces values not found in S.
-- A deletion or update to S causes some tuples of R to “dangle.”
Sells (R)
Beers (S)
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
Bud Lite
3.20
name
manf
Bud
A.B.
Miller
MBC
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
name
manf
Bud
A.B.
Miller
MBC
No Bud Lite

38. Enforcing Foreign-Key Constraints
Sells(bar, beer, price)
Beers(name, manf)
Enforcing Foreign-Key Constraints
If there is a foreign-key constraint from attributes of relation R to a key of relation S, two violations are possible:
-- An insert or update to R introduces values not found in S.
-- A deletion or update to S causes some tuples of R to “dangle.”
Sells (R)
Beers (S)
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
Bud Lite
3.20
name
manf
Bud
A.B.
Miller
MBC
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
name
manf
Bud
A.B.
Miller
MBC
No Bud Lite

39. Enforcing Foreign-Key Constraints
Sells(bar, beer, price)
Beers(name, manf)
Enforcing Foreign-Key Constraints
If there is a foreign-key constraint from attributes of relation R to a key of relation S, two violations are possible:
-- An insert or update to R introduces values not found in S.
-- A deletion or update to S causes some tuples of R to “dangle.”
Sells (R)
Beers (S)
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
Bud Lite
3.20
name
manf
Bud
A.B.
Miller
MBC
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
name
manf
Bud
A.B.
Miller
MBC ?
No Bud Lite

40. Actions Taken

41. Actions Taken Suppose R = Sells, S = Beers. Sells(bar, beer, price)
Beers(name, manf)
Actions Taken
Suppose R = Sells, S = Beers.

42. Actions Taken Suppose R = Sells, S = Beers.
Sells(bar, beer, price)
Beers(name, manf)
Actions Taken
Suppose R = Sells, S = Beers.
An insert or update to Sells that introduces a nonexistent beer must be rejected.

43. Actions Taken Suppose R = Sells, S = Beers.
Sells(bar, beer, price)
Beers(name, manf)
Actions Taken
Suppose R = Sells, S = Beers.
An insert or update to Sells that introduces a nonexistent beer must be rejected.
A deletion or update to Beers that removes a beer value found in some tuples of Sells can be handled in three ways:

44. Actions Taken Suppose R = Sells, S = Beers.
Sells(bar, beer, price)
Beers(name, manf)
Actions Taken
Suppose R = Sells, S = Beers.
An insert or update to Sells that introduces a nonexistent beer must be rejected.
A deletion or update to Beers that removes a beer value found in some tuples of Sells can be handled in three ways:
-- Default: Reject the modification.

45. Actions Taken Suppose R = Sells, S = Beers.
Sells(bar, beer, price)
Beers(name, manf)
Actions Taken
Suppose R = Sells, S = Beers.
An insert or update to Sells that introduces a nonexistent beer must be rejected.
A deletion or update to Beers that removes a beer value found in some tuples of Sells can be handled in three ways:
-- Default: Reject the modification.
-- Cascade: Make the same changes in Sells.

46. Actions Taken Suppose R = Sells, S = Beers.
Sells(bar, beer, price)
Beers(name, manf)
Actions Taken
Suppose R = Sells, S = Beers.
An insert or update to Sells that introduces a nonexistent beer must be rejected.
A deletion or update to Beers that removes a beer value found in some tuples of Sells can be handled in three ways:
-- Default: Reject the modification.
-- Cascade: Make the same changes in Sells.
* Deleted beer: delete Sells tuple.

47. Actions Taken Suppose R = Sells, S = Beers.
Sells(bar, beer, price)
Beers(name, manf)
Actions Taken
Suppose R = Sells, S = Beers.
An insert or update to Sells that introduces a nonexistent beer must be rejected.
A deletion or update to Beers that removes a beer value found in some tuples of Sells can be handled in three ways:
-- Default: Reject the modification.
-- Cascade: Make the same changes in Sells.
* Deleted beer: delete Sells tuple.
* Updated beer: change value in Sells.

48. Actions Taken Suppose R = Sells, S = Beers.
Sells(bar, beer, price)
Beers(name, manf)
Actions Taken
Suppose R = Sells, S = Beers.
An insert or update to Sells that introduces a nonexistent beer must be rejected.
A deletion or update to Beers that removes a beer value found in some tuples of Sells can be handled in three ways:
-- Default: Reject the modification.
-- Cascade: Make the same changes in Sells.
* Deleted beer: delete Sells tuple.
* Updated beer: change value in Sells.
-- Set NULL: Change the beer (in Sells) to NULL.

49. Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade

50. Example: Cascade Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade
Delete the Bud tuple from Beers:

51. Example: Cascade Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade
Delete the Bud tuple from Beers:
-- Then delete all tuples from Sells that have beer = ’Bud’.

52. Example: Cascade Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade
Delete the Bud tuple from Beers:
-- Then delete all tuples from Sells that have beer = ’Bud’.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC

53. Example: Cascade Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade
Delete the Bud tuple from Beers:
-- Then delete all tuples from Sells that have beer = ’Bud’.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC

54. Example: Cascade Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade
Delete the Bud tuple from Beers:
-- Then delete all tuples from Sells that have beer = ’Bud’.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC

55. Example: Cascade Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade
Delete the Bud tuple from Beers:
-- Then delete all tuples from Sells that have beer = ’Bud’.
Update the Bud tuple by changing ’Bud’ to ’Budweiser’:
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC

56. Example: Cascade Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade
Delete the Bud tuple from Beers:
-- Then delete all tuples from Sells that have beer = ’Bud’.
Update the Bud tuple by changing ’Bud’ to ’Budweiser’:
-- Then change all Sells tuples with beer = ’Bud’ so that beer = ’Budweiser’.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC

57. Example: Cascade Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade
Delete the Bud tuple from Beers:
-- Then delete all tuples from Sells that have beer = ’Bud’.
Update the Bud tuple by changing ’Bud’ to ’Budweiser’:
-- Then change all Sells tuples with beer = ’Bud’ so that beer = ’Budweiser’.
Sells (R)
Beers (S)
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC

58. Example: Cascade Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade
Delete the Bud tuple from Beers:
-- Then delete all tuples from Sells that have beer = ’Bud’.
Update the Bud tuple by changing ’Bud’ to ’Budweiser’:
-- Then change all Sells tuples with beer = ’Bud’ so that beer = ’Budweiser’.
Sells (R)
Beers (S)
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
Budweiser

59. Example: Cascade Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Cascade
Delete the Bud tuple from Beers:
-- Then delete all tuples from Sells that have beer = ’Bud’.
Update the Bud tuple by changing ’Bud’ to ’Budweiser’:
-- Then change all Sells tuples with beer = ’Bud’ so that beer = ’Budweiser’.
Sells (R)
Beers (S)
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
Budweiser
Budweiser
Budweiser

60. Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL

61. Example: Set NULL Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL
Delete the Bud tuple from Beers:

62. Example: Set NULL Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL
Delete the Bud tuple from Beers:
-- Change all tuples of Sells that have beer = ’Bud’ to have beer = NULL.

63. Example: Set NULL Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL
Delete the Bud tuple from Beers:
-- Change all tuples of Sells that have beer = ’Bud’ to have beer = NULL.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC

64. Example: Set NULL Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL
Delete the Bud tuple from Beers:
-- Change all tuples of Sells that have beer = ’Bud’ to have beer = NULL.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC

65. Example: Set NULL Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL
Delete the Bud tuple from Beers:
-- Change all tuples of Sells that have beer = ’Bud’ to have beer = NULL.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
NULL
NULL

66. Example: Set NULL Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL
Delete the Bud tuple from Beers:
-- Change all tuples of Sells that have beer = ’Bud’ to have beer = NULL.
Update the Bud tuple by changing ’Bud’ to ’Budweiser’:
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
NULL
NULL

67. Example: Set NULL Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL
Delete the Bud tuple from Beers:
-- Change all tuples of Sells that have beer = ’Bud’ to have beer = NULL.
Update the Bud tuple by changing ’Bud’ to ’Budweiser’:
-- the same change as above.
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
NULL
NULL

68. Example: Set NULL Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL
Delete the Bud tuple from Beers:
-- Change all tuples of Sells that have beer = ’Bud’ to have beer = NULL.
Update the Bud tuple by changing ’Bud’ to ’Budweiser’:
-- the same change as above.
Sells (R)
Beers (S)
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
NULL
NULL

69. Example: Set NULL Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL
Delete the Bud tuple from Beers:
-- Change all tuples of Sells that have beer = ’Bud’ to have beer = NULL.
Update the Bud tuple by changing ’Bud’ to ’Budweiser’:
-- the same change as above.
Sells (R)
Beers (S)
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
NULL
Budweiser
NULL

70. Example: Set NULL Delete the Bud tuple from Beers:
Sells(bar, beer, price)
Beers(name, manf)
Example: Set NULL
Delete the Bud tuple from Beers:
-- Change all tuples of Sells that have beer = ’Bud’ to have beer = NULL.
Update the Bud tuple by changing ’Bud’ to ’Budweiser’:
-- the same change as above.
Sells (R)
Beers (S)
Sells (R)
Beers (S)
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
bar
beer
price
Joe’s
Bud
3.00
Sue’s
Miller
3.50
3.20
name
manf
Bud
A.B.
Miller
MBC
NULL
NULL
Budweiser
NULL
NULL

71. Choosing a Policy

72. Choosing a Policy
When we declare a foreign key, we may choose policies SET NULL or CASCADE independently for deletions and updates.

73. Choosing a Policy
When we declare a foreign key, we may choose policies SET NULL or CASCADE independently for deletions and updates.
Follow the foreign-key declaration by:
ON [UPDATE, DELETE][SET NULL, CASCADE]

74. Choosing a Policy
When we declare a foreign key, we may choose policies SET NULL or CASCADE independently for deletions and updates.
Follow the foreign-key declaration by:
ON [UPDATE, DELETE][SET NULL, CASCADE]
Two such clauses may be used.

75. Choosing a Policy
When we declare a foreign key, we may choose policies SET NULL or CASCADE independently for deletions and updates.
Follow the foreign-key declaration by:
ON [UPDATE, DELETE][SET NULL, CASCADE]
Two such clauses may be used.
Otherwise, the default (reject) is used.

76. Example CREATE TABLE Sells ( bar CHAR(20), beer CHAR(20), price REAL,
Sells(bar, beer, price)
Beers(name, manf)
Example
CREATE TABLE Sells (
bar CHAR(20),
beer CHAR(20),
price REAL,
FOREIGN KEY(beer)
REFERENCES Beers(name)
ON DELETE SET NULL
ON UPDATE CASCADE );

77. Attribute-Based Checks

78. Attribute-Based Checks
Constraints on the value of an attribute.

79. Attribute-Based Checks
Constraints on the value of an attribute.
Add: CHECK (<condition>) to the declaration for the attribute.

80. Attribute-Based Checks
Constraints on the value of an attribute.
Add: CHECK (<condition>) to the declaration for the attribute.
The condition may use the name of the attribute, but any other relation or attribute name must be in a subquery.

81. Example: Attribute-Based Check
Beers(name, manf)
Sells(bar, beer, price)
Example: Attribute-Based Check
CREATE TABLE Sells (
bar CHAR(20),
beer CHAR(20)
CHECK ( beer IN
(SELECT name FROM Beers)),
price REAL
CHECK ( price <= 5.00 ) );

82. Timing of Attribute-based Checks

83. Timing of Attribute-based Checks
Attribute-based checks performed only when a value for that attribute is inserted or updated.

84. Timing of Attribute-based Checks
Attribute-based checks performed only when a value for that attribute is inserted or updated.
-- Example: CHECK (price <= 5.00) checks every new price and rejects the modification (for that tuple) if the price is more than $5.

85. Timing of Attribute-based Checks
Attribute-based checks performed only when a value for that attribute is inserted or updated.
-- Example: CHECK (price <= 5.00) checks every new price and rejects the modification (for that tuple) if the price is more than $5.
-- Example: (in the relation Sells(bar, beer, price))
CHECK (beer IN (SELECT name FROM Beers))

86. Timing of Attribute-based Checks
Attribute-based checks performed only when a value for that attribute is inserted or updated.
-- Example: CHECK (price <= 5.00) checks every new price and rejects the modification (for that tuple) if the price is more than $5.
-- Example: (in the relation Sells(bar, beer, price))
CHECK (beer IN (SELECT name FROM Beers))
is not checked if a beer is deleted from Beers

87. Timing of Attribute-based Checks
Attribute-based checks performed only when a value for that attribute is inserted or updated.
-- Example: CHECK (price <= 5.00) checks every new price and rejects the modification (for that tuple) if the price is more than $5.
-- Example: (in the relation Sells(bar, beer, price))
CHECK (beer IN (SELECT name FROM Beers))
is not checked if a beer is deleted from Beers it is only checked for Sells (unlike foreign-keys).

88. Tuple-Based Checks

89. Tuple-Based Checks
CHECK (<condition>) may be added as a relation-schema element.

90. Tuple-Based Checks
CHECK (<condition>) may be added as a relation-schema element.
The condition may refer to any attribute of the relation (but any other attributes or relations require a subquery).

91. Tuple-Based Checks
CHECK (<condition>) may be added as a relation-schema element.
The condition may refer to any attribute of the relation (but any other attributes or relations require a subquery).
Checked on insert or update only.

92. Example: Tuple-Based Check
Sells(bar, beer, price)
Example: Tuple-Based Check
Only Joe’s Bar can sell beer for more than $5
CREATE TABLE Sells (
bar CHAR(20),
beer CHAR(20),
price REAL,
CHECK (bar = ’Joe’’s Bar’ OR
price <= 5.00) );

93. Cross-Relation Constraints

94. Cross-Relation Constraints
Remark. All constraints we discussed so far are given in the declaration of a relation, i.e., in a CREATE TABLE statement.

95. Cross-Relation Constraints
Remark. All constraints we discussed so far are given in the declaration of a relation, i.e., in a CREATE TABLE statement.
Constraints can also be given as database- schema elements, like relations or views.

96. Cross-Relation Constraints
Remark. All constraints we discussed so far are given in the declaration of a relation, i.e., in a CREATE TABLE statement.
Constraints can also be given as database- schema elements, like relations or views.
Assertions:
CREATE ASSERTION <name>
CHECK (<condition>);

97. Cross-Relation Constraints
Remark. All constraints we discussed so far are given in the declaration of a relation, i.e., in a CREATE TABLE statement.
Constraints can also be given as database- schema elements, like relations or views.
Assertions:
CREATE ASSERTION <name>
CHECK (<condition>);
Condition may refer to any relation or attribute in the database schema.

98. Sells(bar, beer, price)
Example 1: Assertion
In Sells(bar, beer, price), no bar may charge an average of more than $5.

99. Sells(bar, beer, price)
Example 1: Assertion
In Sells(bar, beer, price), no bar may charge an average of more than $5.
CREATE ASSERTION NoRipoffBars
CHECK (
NOT EXISTS (
SELECT bar FROM Sells
GROUP BY bar
HAVING 5.00 < AVG(price)) );
bars with an
average price
above $5

100. Sells(bar, beer, price)
Example 1: Assertion
In Sells(bar, beer, price), no bar may charge an average of more than $5.
CREATE ASSERTION NoRipoffBars
CHECK (
NOT EXISTS (
SELECT bar FROM Sells
GROUP BY bar
HAVING 5.00 < AVG(price)) );
no bar can have
an average price
larger than $5
bars with an
average price
above $5

101. Sells(bar, beer, price)
Example 1: Assertion
In Sells(bar, beer, price), no bar may charge an average of more than $5.
CREATE ASSERTION NoRipoffBars
CHECK (
NOT EXISTS (
SELECT bar FROM Sells
GROUP BY bar
HAVING 5.00 < AVG(price)) );
no bar can have
an average price
larger than $5
bars with an
average price
above $5

102. Sells(bar, beer, price)
Example 1: Assertion
In Sells(bar, beer, price), no bar may charge an average of more than $5.
CREATE ASSERTION NoRipoffBars
CHECK (
NOT EXISTS (
SELECT bar FROM Sells
GROUP BY bar
HAVING 5.00 < AVG(price)) );
no bar can have
an average price
larger than $5
bars with an
average price
above $5
An constraint that no bar can have
an average price larger than $5

103. Sells(bar, beer, price)
Example 1: Assertion
In Sells(bar, beer, price), no bar may charge an average of more than $5.
CREATE ASSERTION NoRipoffBars
CHECK (
NOT EXISTS (
SELECT bar FROM Sells
GROUP BY bar
HAVING 5.00 < AVG(price)) );
An constraint that no bar can have
an average price larger than $5

104. Drinkers(name, addr, phone)
Bars(name, addr, license)
Example 2: Assertion
In Drinkers(name, addr, phone) and Bars(name, addr, license), there cannot be more bars than drinkers.

105. Drinkers(name, addr, phone)
Bars(name, addr, license)
Example 2: Assertion
In Drinkers(name, addr, phone) and Bars(name, addr, license), there cannot be more bars than drinkers.
CREATE ASSERTION FewBar CHECK (
(SELECT COUNT(*) FROM Bars) <=
(SELECT COUNT(*) FROM Drinkers) );

106. Timing of Assertion Checks

107. Timing of Assertion Checks
In principle, we must check every assertion after every modification to any relation of the database.

108. Timing of Assertion Checks
In principle, we must check every assertion after every modification to any relation of the database.
A clever system can observe that only certain changes could cause a given assertion to be violated.

109. Timing of Assertion Checks
In principle, we must check every assertion after every modification to any relation of the database.
A clever system can observe that only certain changes could cause a given assertion to be violated.
-- Example: No change to Beers can affect FewBar Neither can an insertion to Drinkers.
CREATE ASSERTION FewBar CHECK (
(SELECT COUNT(*) FROM Bars) <=
(SELECT COUNT(*) FROM Drinkers) );