1. CPSC-608 Database Systems
Fall 2017
Instructor: Jianer Chen
Office: HRBB 315C
Phone:
Notes 5

2. SQL: Structured Query language
a very-high-level language. * say “what to do” rather than “how to do it.” * avoid a lot of data-manipulation details needed in procedural languages like C or Java.
Database management system figures out the “best” way to execute queries * called “query optimization”
For both data definition and data manipulation.

3. Meaning of A Query Begin with the relation in the FROM clause.
Apply the selection indicated by the WHERE clause.
Group the tuples selected in step 2 in terms of the values of the attributes in GROUP BY.
Apply the projection in the SELECT clause.
SELECT attributes
FROM tables
WHERE conditions
GROUP BY attributes
Step 4
Step 1
Step 2
Step 3

4. HAVING Clauses HAVING <condition> may follow a GROUP BY clause.
If so, the condition applies to each group, and groups not satisfying the condition are eliminated.

5. Sells(bar, beer, price)
Beers(name, manf)
Example. From Sells and Beers, find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s.

6. Sells(bar, beer, price)
Beers(name, manf)
Example. From Sells and Beers, find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s.
SELECT beer, AVG(price)
FROM Sells
GROUP BY beer

7. Sells(bar, beer, price)
Beers(name, manf)
Example. From Sells and Beers, find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s.
group tuples (bar, beer, price) in Sells in terms of beer
SELECT beer, AVG(price)
FROM Sells
GROUP BY beer

8. Sells(bar, beer, price)
Beers(name, manf)
Example. From Sells and Beers, find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s.
group tuples (bar, beer, price) in Sells in terms of beer
SELECT beer, AVG(price)
FROM Sells
GROUP BY beer
HAVING COUNT(bar) >= 3 beer IN (SELECT name
FROM Beers
WHERE manf = ’Pete’’s’);
at least 3 bars appear in the beer group

9. Sells(bar, beer, price)
Beers(name, manf)
Example. From Sells and Beers, find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s.
group tuples (bar, beer, price) in Sells in terms of beer
SELECT beer, AVG(price)
FROM Sells
GROUP BY beer
HAVING COUNT(bar) >= 3 OR
beer IN (SELECT name
FROM Beers
WHERE manf = ’Pete’’s’);
at least 3 bars appear in the beer group

10. Sells(bar, beer, price)
Beers(name, manf)
Example. From Sells and Beers, find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s.
group tuples (bar, beer, price) in Sells in terms of beer
SELECT beer, AVG(price)
FROM Sells
GROUP BY beer
HAVING COUNT(bar) >= 3 OR
beer IN (SELECT name
FROM Beers
WHERE manf = ’Pete’’s’);
at least 3 bars appear in the beer group

11. Sells(bar, beer, price)
Beers(name, manf)
Example. From Sells and Beers, find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s.
group tuples (bar, beer, price) in Sells in terms of beer
SELECT beer, AVG(price)
FROM Sells
GROUP BY beer
HAVING COUNT(bar) >= 3 OR
beer IN (SELECT name
FROM Beers
WHERE manf = ’Pete’’s’);
at least 3 bars appear in the beer group
beers made by Pete’s

12. Sells(bar, beer, price)
Beers(name, manf)
Example. From Sells and Beers, find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s.
group tuples (bar, beer, price) in Sells in terms of beer
SELECT beer, AVG(price)
FROM Sells
GROUP BY beer
HAVING COUNT(bar) >= 3 OR
beer IN (SELECT name
FROM Beers
WHERE manf = ’Pete’’s’);
at least 3 bars appear in the beer group
the beer is made by Pete’s
beers made by Pete’s

13. Requirements on HAVING Conditions
These conditions may refer to any relation or tuple-variable in the FROM clause.

14. Requirements on HAVING Conditions
These conditions may refer to any relation or tuple-variable in the FROM clause.
They may refer to attributes of those relations, as long as the attribute makes sense within a group; i.e., it is either:
A grouping attribute, or
Aggregated.

15. Requirements on HAVING Conditions
It is easier to understand this from an implementation viewpoint:
SELECT
FROM
WHERE
GROUP BY
HAVING

16. Requirements on HAVING Conditions
It is easier to understand this from an implementation viewpoint:
SELECT
FROM
WHERE
GROUP BY
HAVING
step 1, input

17. Requirements on HAVING Conditions
It is easier to understand this from an implementation viewpoint:
SELECT
FROM
WHERE
GROUP BY
HAVING
step 1, input
step 2, pick the proper tuples

18. Requirements on HAVING Conditions
It is easier to understand this from an implementation viewpoint:
SELECT
FROM
WHERE
GROUP BY
HAVING
step 1, input
step 2, pick the proper tuples
step 3, group the picked tuples

19. Requirements on HAVING Conditions
It is easier to understand this from an implementation viewpoint:
SELECT
FROM
WHERE
GROUP BY
HAVING
step 1, input
step 2, pick the proper tuples
step 3, group the picked tuples
step 4, pick the proper groups

20. Requirements on HAVING Conditions
It is easier to understand this from an implementation viewpoint:
SELECT
FROM
WHERE
GROUP BY
HAVING
step 5, compute the output
step 1, input
step 2, pick the proper tuples
step 3, group the picked tuples
step 4, pick the proper groups

21. Database Modifications

22. Database Modifications
A modification command does not return a result (as a query does), but changes the database in some way.

23. Database Modifications
A modification command does not return a result (as a query does), but changes the database in some way.
Three kinds of modifications:
Insert a tuple or tuples.
Delete a tuple or tuples.
Update the value(s) of an existing tuple or tuples.

24. Insertion

25. Insertion To insert a single tuple:
INSERT INTO <relation> VALUES (<list of values>);

26. Insertion To insert a single tuple:
Likes(drinker, beer)
Insertion
To insert a single tuple:
INSERT INTO <relation> VALUES (<list of values>);
Example: add to Likes(drinker, beer) the fact that Sally likes Bud.
INSERT INTO Likes VALUES(’Sally’, ’Bud’);

27. Insertion To insert a single tuple:
Likes(drinker, beer)
Insertion
To insert a single tuple:
INSERT INTO <relation> VALUES (<list of values>);
Example: add to Likes(drinker, beer) the fact that Sally likes Bud.
INSERT INTO Likes VALUES(’Sally’, ’Bud’);
We may add a list of attributes to <relation>.

28. Insertion To insert a single tuple:
Likes(drinker, beer)
Insertion
To insert a single tuple:
INSERT INTO <relation> VALUES (<list of values>);
Example: add to Likes(drinker, beer) the fact that Sally likes Bud.
INSERT INTO Likes VALUES(’Sally’, ’Bud’);
We may add a list of attributes to <relation>. Two reasons for doing so:
Forget the order of attributes for the relation.
Don’t have values for all attributes, and want the system to fill in missing ones with default values.

29. Insertion To insert a single tuple:
Likes(drinker, beer)
Insertion
To insert a single tuple:
INSERT INTO <relation> VALUES (<list of values>);
Example: add to Likes(drinker, beer) the fact that Sally likes Bud.
INSERT INTO Likes VALUES(’Sally’, ’Bud’);
We may add a list of attributes to <relation>. Two reasons for doing so:
Forget the order of attributes for the relation.
Don’t have values for all attributes, and want the system to fill in missing ones with default values.
So another solution for the above example:
INSERT INTO Likes(beer, drinker) VALUES(‘Bud’, ’Sally’);

30. Inserting Many Tuples
We may insert the entire result of a query into a relation, using the form:
INSERT INTO <relation>
(<subquery>);

31. Frequents(drinker, bar)
Example. Using Frequents, enter into the new relation PotBuddies(name) all of Sally’s “potential buddies,” i.e., those drinkers who frequent at least one bar that Sally also frequents.

32. Frequents(drinker, bar)
Example. Using Frequents, enter into the new relation PotBuddies(name) all of Sally’s “potential buddies,” i.e., those drinkers who frequent at least one bar that Sally also frequents.
1. find all potential buddies of Sally by pairing Sally with those who frequent the bars Sally frequents.
INSERT INTO PotBuddies
(SELECT d2.drinker
FROM Frequents d1, Frequents d2
WHERE d1.drinker = ’Sally’ AND
d2.drinker <> ’Sally’ AND
d1.bar = d2.bar);

33. Frequents(drinker, bar)
Example. Using Frequents, enter into the new relation PotBuddies(name) all of Sally’s “potential buddies,” i.e., those drinkers who frequent at least one bar that Sally also frequents.
1. find all potential buddies of Sally by pairing Sally with those who frequent the bars Sally frequents.
INSERT INTO PotBuddies
(SELECT d2.drinker
FROM Frequents d1, Frequents d2
WHERE d1.drinker = ’Sally’ AND
d2.drinker <> ’Sally’ AND
d1.bar = d2.bar);
(Sally, Joe’s, Tom, Joe’s)
(Sally, Sue’s, Jeff, Sue’s)
(Sally, Sue’s, Mary, Sue’s) ……

34. Frequents(drinker, bar)
Example. Using Frequents, enter into the new relation PotBuddies(name) all of Sally’s “potential buddies,” i.e., those drinkers who frequent at least one bar that Sally also frequents.
1. find all potential buddies of Sally by pairing Sally with those who frequent the bars Sally frequents.
2. collect the drinkers
INSERT INTO PotBuddies
(SELECT d2.drinker
FROM Frequents d1, Frequents d2
WHERE d1.drinker = ’Sally’ AND
d2.drinker <> ’Sally’ AND
d1.bar = d2.bar);
(Sally, Joe’s, Tom, Joe’s)
(Sally, Sue’s, Jeff, Sue’s)
(Sally, Sue’s, Mary, Sue’s) ……

35. Frequents(drinker, bar)
Example. Using Frequents, enter into the new relation PotBuddies(name) all of Sally’s “potential buddies,” i.e., those drinkers who frequent at least one bar that Sally also frequents.
1. find all potential buddies of Sally by pairing Sally with those who frequent the bars Sally frequents.
2. collect the drinkers
INSERT INTO PotBuddies
(SELECT d2.drinker
FROM Frequents d1, Frequents d2
WHERE d1.drinker = ’Sally’ AND
d2.drinker <> ’Sally’ AND
d1.bar = d2.bar);
Tom
Jeff
Mary …

36. Frequents(drinker, bar)
Example. Using Frequents, enter into the new relation PotBuddies(name) all of Sally’s “potential buddies,” i.e., those drinkers who frequent at least one bar that Sally also frequents.
1. find all potential buddies of Sally by pairing Sally with those who frequent the bars Sally frequents.
2. collect the drinkers
INSERT INTO PotBuddies
(SELECT d2.drinker
FROM Frequents d1, Frequents d2
WHERE d1.drinker = ’Sally’ AND
d2.drinker <> ’Sally’ AND
d1.bar = d2.bar);
Tom
Jeff
Mary …
3. add the drinkers to PotBuddies

37. Frequents(drinker, bar)
Example. Using Frequents, enter into the new relation PotBuddies(name) all of Sally’s “potential buddies,” i.e., those drinkers who frequent at least one bar that Sally also frequents.
1. find all potential buddies of Sally by pairing Sally with those who frequent the bars Sally frequents.
2. collect the drinkers
PotBuddies
D2.drinker
Tom
Jeff
Mary …
INSERT INTO PotBuddies
(SELECT d2.drinker
FROM Frequents d1, Frequents d2
WHERE d1.drinker = ’Sally’ AND
d2.drinker <> ’Sally’ AND
d1.bar = d2.bar);
3. add the drinkers to PotBuddies

38. Deletion

39. Deletion To delete tuples satisfying a condition from some relation:
DELETE FROM <relation>
WHERE <condition>;

40. Deletion To delete tuples satisfying a condition from some relation:
Likes(drinker, beer)
Deletion
To delete tuples satisfying a condition from some relation:
DELETE FROM <relation>
WHERE <condition>;
Example. Delete from Likes the fact that Sally likes Bud:

41. Deletion To delete tuples satisfying a condition from some relation:
Likes(drinker, beer)
Deletion
To delete tuples satisfying a condition from some relation:
DELETE FROM <relation>
WHERE <condition>;
Example. Delete from Likes the fact that Sally likes Bud:
DELETE FROM Likes
WHERE drinker = ’Sally’ AND beer = ’Bud’;

42. Deletion To delete tuples satisfying a condition from some relation:
Likes(drinker, beer)
Deletion
To delete tuples satisfying a condition from some relation:
DELETE FROM <relation>
WHERE <condition>;
Example. Delete from Likes the fact that Sally likes Bud:
DELETE FROM Likes
WHERE drinker = ’Sally’ AND beer = ’Bud’;
To make the relation Likes empty:
Note that no WHERE clause is needed

43. Example: Delete Many Tuples
Delete from Beers(name, manf) all beers for which there is another beer by the same manufacturer.

44. Example: Delete Many Tuples
Delete from Beers(name, manf) all beers for which there is another beer by the same manufacturer.
DELETE FROM Beers b
WHERE EXISTS (
SELECT name FROM Beers
WHERE manf = b.manf AND
name <> b.name);

45. Example: Delete Many Tuples
Delete from Beers(name, manf) all beers for which there is another beer by the same manufacturer.
DELETE FROM Beers b
WHERE EXISTS (
SELECT name FROM Beers
WHERE manf = b.manf AND
name <> b.name);
Beers with the same manufacturer and a different name from the name of the beer represented by tuple b.

46. Semantics of Deletion

47. name
manf
Bud
Anheuser-Busch
Bud Lite
Semantics of Deletion
Suppose Anheuser-Busch makes only Bud and Bud Lite.

48. name
manf
Bud
Anheuser-Busch
Bud Lite
Semantics of Deletion
Suppose Anheuser-Busch makes only Bud and Bud Lite.
If we come to the tuple b for Bud first.

49. name
manf
Bud
Anheuser-Busch
Bud Lite
Semantics of Deletion
Suppose Anheuser-Busch makes only Bud and Bud Lite.
If we come to the tuple b for Bud first.
The subquery is nonempty, because of the Bud Lite tuple, so we delete Bud.

50. name
manf
Bud
Anheuser-Busch
Bud Lite
Semantics of Deletion
Suppose Anheuser-Busch makes only Bud and Bud Lite.
If we come to the tuple b for Bud first.
The subquery is nonempty, because of the Bud Lite tuple, so we delete Bud.

51. name
manf
Bud
Anheuser-Busch
Bud Lite
Semantics of Deletion ?
Suppose Anheuser-Busch makes only Bud and Bud Lite.
If we come to the tuple b for Bud first.
The subquery is nonempty, because of the Bud Lite tuple, so we delete Bud. (?)

52. name
manf
Bud
Anheuser-Busch
Bud Lite
Semantics of Deletion ?
Suppose Anheuser-Busch makes only Bud and Bud Lite.
If we come to the tuple b for Bud first.
The subquery is nonempty, because of the Bud Lite tuple, so we delete Bud. (?)
Now, when b is the tuple for Bud Lite, do we delete that tuple too? ?

53. name
manf
Bud
Anheuser-Busch
Bud Lite
Semantics of Deletion
Suppose Anheuser-Busch makes only Bud and Bud Lite.
If we come to the tuple b for Bud first.
The subquery is nonempty, because of the Bud Lite tuple, so we delete Bud. (?)
Now, when b is the tuple for Bud Lite, do we delete that tuple too?
Answer: we do delete Bud Lite as well.

54. name
manf
Bud
Anheuser-Busch
Bud Lite
Semantics of Deletion
Suppose Anheuser-Busch makes only Bud and Bud Lite.
If we come to the tuple b for Bud first.
The subquery is nonempty, because of the Bud Lite tuple, so we delete Bud. (?)
Now, when b is the tuple for Bud Lite, do we delete that tuple too?
Answer: we do delete Bud Lite as well.
Reason: Deletion proceeds in two stages:

55. name
manf
Bud
Anheuser-Busch
Bud Lite
Semantics of Deletion
Suppose Anheuser-Busch makes only Bud and Bud Lite.
If we come to the tuple b for Bud first.
The subquery is nonempty, because of the Bud Lite tuple, so we delete Bud. (?)
Now, when b is the tuple for Bud Lite, do we delete that tuple too?
Answer: we do delete Bud Lite as well.
Reason: Deletion proceeds in two stages:
-- Mark all tuples for which WHERE condition holds.

56. name
manf
Bud
Anheuser-Busch
Bud Lite
Semantics of Deletion
Suppose Anheuser-Busch makes only Bud and Bud Lite.
If we come to the tuple b for Bud first.
The subquery is nonempty, because of the Bud Lite tuple, so we delete Bud. (?)
Now, when b is the tuple for Bud Lite, do we delete that tuple too?
Answer: we do delete Bud Lite as well.
Reason: Deletion proceeds in two stages:
-- Mark all tuples for which WHERE condition holds.
-- Delete the marked tuples.

57. Updates

58. Updates To change certain attributes in certain tuples of a relation:
UPDATE <relation>
SET <list of attribute assignments>
WHERE <condition on tuples>;

59. Updates To change certain attributes in certain tuples of a relation:
Drinkers(name, addr, phone)
Updates
To change certain attributes in certain tuples of a relation:
UPDATE <relation>
SET <list of attribute assignments>
WHERE <condition on tuples>;
Change Fred’s phone number to :
UPDATE Drinkers
SET phone = ’ ’
WHERE name = ’Fred’;

60. Updates To change certain attributes in certain tuples of a relation:
Drinkers(name, addr, phone)
Sells(bar, beer, price)
Updates
To change certain attributes in certain tuples of a relation:
UPDATE <relation>
SET <list of attribute assignments>
WHERE <condition on tuples>;
Change Fred’s phone number to :
UPDATE Drinkers
SET phone = ’ ’
WHERE name = ’Fred’;
Make $4 the maximum price for beer
(make updates for several tuples):
UPDATE Sells
SET price = 4.00
WHERE price > 4.00;

61. Constraints and Triggers

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

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

64. 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.

65. 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.

66. 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.

67. Kinds of Constraints

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

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

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

71. 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.

72. 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.

73. 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.

74. 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.

75. Expressing Foreign Keys

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

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

78. 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>)

79. 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>.

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

81. 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 );

82. 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 );

83. 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) );

84. 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));

85. 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));

86. 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

87. 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

88. 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:

89. 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.

90. 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

91. 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

92. 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

93. 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

94. 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

95. 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

96. Actions Taken

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

98. 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.

99. 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:

100. 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.

101. 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.

102. 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.

103. 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.

104. 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.

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

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

107. 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’.

108. 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

109. 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

110. 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

111. 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

112. 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

113. 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

114. 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

115. 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

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

117. 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:

118. 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.

119. 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

120. 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

121. 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

122. 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

123. 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

124. 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

125. 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

126. 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

127. Choosing a Policy

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

129. 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]

130. 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.

131. 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.

132. 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 );

133. Attribute-Based Checks

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

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

136. 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.

137. 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 ) );

138. Timing of Attribute-based Checks

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

140. 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.

141. 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))

142. 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

143. 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).

144. Tuple-Based Checks

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

146. 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).

147. 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.

148. 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) );

149. Cross-Relation Constraints

150. 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.

151. 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.

152. 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>);

153. 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.

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

155. 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

156. 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

157. 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

158. 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

159. 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

160. 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.

161. 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) );

162. Timing of Assertion Checks

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

164. 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.

165. 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) );