1. CPSC-310 Database Systems
Professor Jianer Chen
Room 315C HRBB
Lecture #12

2. SQL: Structured Query language
How does SQL manipulate (search in/read from/write to) tables?

3. SQL: Structured Query language
How does SQL manipulate (search in/read from/write to) tables?
This is the job of a data manipulation language (DML). DML does not change database schema.

4. Select-From-Where Statements
SELECT attributes
FROM tables
WHERE conditions

5. Select-From-Where Statements
SELECT attributes
FROM tables
WHERE conditions
input

6. Select-From-Where Statements
SELECT attributes
FROM tables
WHERE conditions
output (as a table)
input

7. Our Running Example
All 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)

8. Example
Using Beers(name, manf), what beers are made by Anheuser-Busch?
SELECT name
FROM Beers
WHERE manf=’Anheuser-Busch’;

9. Example
Using Beers(name, manf), what beers are made by Anheuser-Busch?
SELECT name
FROM Beers
WHERE manf=’Anheuser-Busch’;
Notice SQL uses single-quotes for strings.
SQL is case-insensitive, except inside strings.

10. Result of Query name Bud Bud Lite Michelob ……
The answer is a relation with a single attribute,
name, and tuples with the name of each beer
by Anheuser-Busch, such as Bud.

11. Meaning of Single-Relation Query

12. Meaning of Single-Relation Query
Begin with the relation in the FROM clause.

13. Meaning of Single-Relation Query
Begin with the relation in the FROM clause.
Apply the selection indicated by the WHERE clause.

14. Meaning of Single-Relation Query
Begin with the relation in the FROM clause.
Apply the selection indicated by the WHERE clause.
Apply the extended projection indicated by the SELECT clause.

15. Meaning of Single-Relation Query
Begin with the relation in the FROM clause.
Apply the selection indicated by the WHERE clause.
Apply the extended projection indicated by the SELECT clause.
SELECT attributes
FROM tables
WHERE conditions

16. Meaning of Single-Relation Query
Begin with the relation in the FROM clause.
Apply the selection indicated by the WHERE clause.
Apply the extended projection indicated by the SELECT clause.
SELECT attributes
FROM tables
WHERE conditions
Step 3
Step 1
Step 2

17. Operational Semantics
Beers
name
manf
Bud
Anheuser-Busch
SELECT name
FROM Beers
WHERE manf=’Anheuser-Busch’;

18. Operational Semantics
Beers
name
manf
Bud
Anheuser-Busch
To implement this algorithm think of a tuple variable tv ranging over each tuple of the relation mentioned in the FROM Clause. tv
SELECT name
FROM Beers
WHERE manf=’Anheuser-Busch’;

19. Operational Semantics
Beers
name
manf
Bud
Anheuser-Busch
To implement this algorithm think of a tuple variable tv ranging over each tuple of the relation mentioned in the FROM Clause.
Check if the “current” tuple satisfies the WHERE clause. tv
Check if manf =
Anheuser-Busch
SELECT name
FROM Beers
WHERE manf=’Anheuser-Busch’;

20. Operational Semantics
Beers
Include tv.name
in the result
name
manf
Bud
Anheuser-Busch
To implement this algorithm think of a tuple variable tv ranging over each tuple of the relation mentioned in the FROM Clause.
Check if the “current” tuple satisfies the WHERE clause.
If so, compute the attributes or expressions of the SELECT clause using the components of this tuple. tv
Check if manf =
Anheuser-Busch
SELECT name
FROM Beers
WHERE manf=’Anheuser-Busch’;

21. * In SELECT clauses

22. * In SELECT clauses
When there is one relation in the FROM clause, * in the SELECT clause stands for “all attributes of this relation.”

23. * In SELECT clauses SELECT *
When there is one relation in the FROM clause, * in the SELECT clause stands for “all attributes of this relation.”
Example using Beers(name, manf):
SELECT *
FROM Beers
WHERE manf = ’Anheuser-Busch’;

24. * In SELECT clauses SELECT *
When there is one relation in the FROM clause, * in the SELECT clause stands for “all attributes of this relation.”
Example using Beers(name, manf):
SELECT *
FROM Beers
WHERE manf = ’Anheuser-Busch’;
The output has the same schema as Beers:
name
manf
Bud
Anheuser-Busch
Bud Lite
Michelob ……

25. Renaming Attributes

26. Renaming Attributes
you want the result to have different attribute names, use “AS <new name>” to rename an attribute.

27. Renaming Attributes SELECT name AS beer, manf
you want the result to have different attribute names, use “AS <new name>” to rename an attribute.
Example using Beers(name, manf):
SELECT name AS beer, manf
FROM Beers
WHERE manf = ’Anheuser-Busch’

28. Renaming Attributes SELECT name AS beer, manf
you want the result to have different attribute names, use “AS <new name>” to rename an attribute.
Example using Beers(name, manf):
SELECT name AS beer, manf
FROM Beers
WHERE manf = ’Anheuser-Busch’
The result is:
beer
manf
Bud
Anheuser-Busch
Bud Lite
Michelob ……

29. Expressions in SELECT Clauses

30. Expressions in SELECT Clauses
Any expression that makes sense can appear as an element of a SELECT clause.

31. Expressions in SELECT Clauses
Any expression that makes sense can appear as an element of a SELECT clause.
Example: from Sells(bar, beer, price):
SELECT bar, beer,
price*114 AS priceInYen
FROM Sells;

32. Expressions in SELECT Clauses
Any expression that makes sense can appear as an element of a SELECT clause.
Example: from Sells(bar, beer, price):
SELECT bar, beer,
price*114 AS priceInYen
FROM Sells;
The result is
bar
beer
priceInYen
Joe’s
Bud
285
Sue’s
Miller
342 ……

33. Expressions in SELECT Clauses
We can also have a “constant expression” in the SELECT clause.

34. Expressions in SELECT Clauses
We can also have a “constant expression” in the SELECT clause.
Example: from Likes(drinker, beer):
SELECT drinker,
‘likes Bud’ AS whoLikesBud
FROM Likes;
WHERE beer = ‘Bud’;

35. Expressions in SELECT Clauses
We can also have a “constant expression” in the SELECT clause.
Example: from Likes(drinker, beer):
SELECT drinker,
‘likes Bud’ AS whoLikesBud
FROM Likes;
WHERE beer = ‘Bud’;
The result is
drinker
whoLikesBud
Sally
likes Bud
Fred ……

36. Complex Conditions in WHERE Clause
From Sells(bar, beer, price), find the price Joe’s Bar charges for Bud:
SELECT price
FROM Sells
WHERE bar = ’Joe’’s Bar’ AND
beer = ’Bud’;

37. Complex Conditions in WHERE Clause
From Sells(bar, beer, price), find the price Joe’s Bar charges for Bud:
SELECT price
FROM Sells
WHERE bar = ’Joe’’s Bar’ AND
beer = ’Bud’;
Notice how we get a single-quote in strings.

38. Patterns
WHERE clauses can have conditions in which a string is compared with a pattern, to check match.

39. Patterns
WHERE clauses can have conditions in which a string is compared with a pattern, to check match.
General form: <Attribute> LIKE <pattern> or <Attribute> NOT LIKE <pattern>

40. Patterns
WHERE clauses can have conditions in which a string is compared with a pattern, to check match.
General form: <Attribute> LIKE <pattern> or <Attribute> NOT LIKE <pattern>
Pattern is a quoted string with % = “any string”; _ = “any character.”

41. Patterns
WHERE clauses can have conditions in which a string is compared with a pattern, to check match.
General form: <Attribute> LIKE <pattern> or <Attribute> NOT LIKE <pattern>
Pattern is a quoted string with % = “any string”; _ = “any character.”
Example. From Drinkers(name, addr, phone), find the drinkers with exchange 555
SELECT name
FROM Drinkers
WHERE phone LIKE ’%555-_ _ _ _’;

42. NULL Values
Tuples in SQL relations can have NULL as a value for one or morecomponents.
Meaning depends on context. Two common cases:
Missing value : e.g., we know Joe’s Bar has some address, but we don’t know what it is.
Inapplicable : e.g., the value of attribute spouse for an unmarried person.

43. Comparing NULL’s to Values
The logic of conditions in SQL is really 3- valued logic: TRUE, FALSE, UNKNOWN.

44. Comparing NULL’s to Values
The logic of conditions in SQL is really 3- valued logic: TRUE, FALSE, UNKNOWN.
When any value is compared with NULL, the truth value is UNKNOWN.

45. Comparing NULL’s to Values
The logic of conditions in SQL is really 3- valued logic: TRUE, FALSE, UNKNOWN.
When any value is compared with NULL, the truth value is UNKNOWN.
But a query only produces a tuple in the answer if its truth value for the WHERE clause is TRUE (not FALSE or UNKNOWN).

46. Three-Valued Logic
To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = 1/2.

47. Three-Valued Logic
To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = 1/2.
AND = MIN; OR = MAX, NOT(x) = 1  x.

48. Three-Valued Logic
To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = 1/2.
AND = MIN; OR = MAX, NOT(x) = 1  x.
Example: TRUE AND (FALSE OR NOT(UNKNOWN))

49. Three-Valued Logic
To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = 1/2.
AND = MIN; OR = MAX, NOT(x) = 1  x.
Example: TRUE AND (FALSE OR NOT(UNKNOWN)) = MIN(1, MAX(0, (11/2)))

50. Three-Valued Logic
To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = 1/2.
AND = MIN; OR = MAX, NOT(x) = 1  x.
Example: TRUE AND (FALSE OR NOT(UNKNOWN)) = MIN(1, MAX(0, (11/2))) = MIN(1, MAX(0, 1/2)

51. Three-Valued Logic
To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = 1/2.
AND = MIN; OR = MAX, NOT(x) = 1  x.
Example: TRUE AND (FALSE OR NOT(UNKNOWN)) = MIN(1, MAX(0, (11/2))) = MIN(1, MAX(0, 1/2) = MIN(1, 1/2)

52. Three-Valued Logic
To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = 1/2.
AND = MIN; OR = MAX, NOT(x) = 1  x.
Example: TRUE AND (FALSE OR NOT(UNKNOWN)) = MIN(1, MAX(0, (11/2))) = MIN(1, MAX(0, 1/2) = MIN(1, 1/2) = 1/2

53. Three-Valued Logic
To understand how AND, OR, and NOT work in 3-valued logic, think of TRUE = 1, FALSE = 0, and UNKNOWN = 1/2.
AND = MIN; OR = MAX, NOT(x) = 1  x.
Example: TRUE AND (FALSE OR NOT(UNKNOWN)) = MIN(1, MAX(0, (11/2))) = MIN(1, MAX(0, 1/2) = MIN(1, 1/2) = 1/2 (= UNKNOWN)

54. Surprising Example

55. Surprising Example From the following Sells relation: Sells bar beer
price
Joe’s Bar
Bud
NULL

56. Surprising Example From the following Sells relation: SELECT bar
beer
price
Joe’s Bar
Bud
NULL
SELECT bar
FROM Sells
WHERE price < 2.00 OR price >= 2.00;

57. Surprising Example From the following Sells relation: SELECT bar
beer
price
Joe’s Bar
Bud
NULL
SELECT bar
FROM Sells
WHERE price < 2.00 OR price >= 2.00;
UNKNOWN
UNKNOWN

58. Surprising Example From the following Sells relation: SELECT bar
beer
price
Joe’s Bar
Bud
NULL
SELECT bar
FROM Sells
WHERE price < 2.00 OR price >= 2.00;
UNKNOWN
UNKNOWN
UNKNOWN

59. Surprising Example From the following Sells relation: SELECT bar
beer
price
Joe’s Bar
Bud
NULL
SELECT bar
FROM Sells
WHERE price < 2.00 OR price >= 2.00;
UNKNOWN
UNKNOWN
UNKNOWN
So Hoe’s Bar will not get printed!

60. Reason: 2-Valued Laws  3-Valued Laws

61. Reason: 2-Valued Laws  3-Valued Laws
Some laws in 2-valued logic, like commutativity of AND:  AND  =  AND , also hold in 3-valued logic.

62. Reason: 2-Valued Laws  3-Valued Laws
Some laws in 2-valued logic, like commutativity of AND:  AND  =  AND , also hold in 3-valued logic.
Not all, e.g., the law of the excluded middle:  OR NOT  = TRUE.

63. Reason: 2-Valued Laws  3-Valued Laws
Some laws in 2-valued logic, like commutativity of AND:  AND  =  AND , also hold in 3-valued logic.
Not all, e.g., the law of the excluded middle:  OR NOT  = TRUE.
Always holds in 2-valued logic

64. Reason: 2-Valued Laws  3-Valued Laws
Some laws in 2-valued logic, like commutativity of AND:  AND  =  AND , also hold in 3-valued logic.
Not all, e.g., the law of the excluded middle:  OR NOT  = TRUE.
Always holds in 2-valued logic
In 3-valued logic, when  = UNKNOWN, the left side is MAX(1/2, (1 – 1/2)) = 1/2  1.

65. Multirelation Queries

66. Multirelation Queries
Interesting queries often combine data from more than one relation.

67. Multirelation Queries
Interesting queries often combine data from more than one relation.
We can address several relations in one query by listing them all in the FROM clause.

68. Multirelation Queries
Interesting queries often combine data from more than one relation.
We can address several relations in one query by listing them all in the FROM clause.
Distinguish attributes of the same name by “<relation>.<attribute>”

69. Example
Using relations Likes(drinker, beer) and Frequents(drinker, bar), find the beers liked by at least one person who frequents Joe’s Bar.

70. Example
Using relations Likes(drinker, beer) and Frequents(drinker, bar), find the beers liked by at least one person who frequents Joe’s Bar.
SELECT beer
FROM Likes, Frequents
WHERE bar = ’Joe’’s Bar’ AND
Frequents.drinker = Likes.drinker;

71. Formal Semantics

72. Formal Semantics Almost the same as for single-relation queries:
SELECT L
FROM R1,R2,…,Rk
WHERE C

73. Formal Semantics Almost the same as for single-relation queries:
Start with the product of all the relations R1, …, Rk in the FROM clause.
SELECT L
FROM R1,R2,…,Rk
WHERE C ╳ R1 R2 …… Rk

74. Formal Semantics Almost the same as for single-relation queries:
Start with the product of all the relations R1, …, Rk in the FROM clause.
Apply the selection condition C from the WHERE clause.
SELECT L
FROM R1,R2,…,Rk
WHERE C σC ╳ R1 R2 …… Rk

75. Formal Semantics Almost the same as for single-relation queries:
Start with the product of all the relations R1, …, Rk in the FROM clause.
Apply the selection condition C from the WHERE clause.
Project onto the list L of attributes and expressions in the SELECT clause.
SELECT L
FROM R1,R2,…,Rk
WHERE C πL σC ╳ R1 R2 …… Rk

76. Formal Semantics Almost the same as for single-relation queries:
Start with the product of all the relations R1, …, Rk in the FROM clause.
Apply the selection condition C from the WHERE clause.
Project onto the list L of attributes and expressions in the SELECT clause.
SELECT L
FROM R1,R2,…,Rk
WHERE C πL σC ╳ R1 R2 …… Rk

77. Operational Semantics
Imagine one tuple-variable for each relation in the FROM clause.
* These tuple-variables visit each combination of tuples, one from each relation.

78. Operational Semantics
Imagine one tuple-variable for each relation in the FROM clause.
* These tuple-variables visit each combination of tuples, one from each relation.
drinker
bar
Sally
Joe’s
drinker
beer
Sally
Bud
tv1
tv2
SELECT beer
FROM Likes, Frequents
WHERE bar = ’Joe’’s Bar’ AND
Frequents.drinker = Likes.drinker;
Frequents
Likes

79. Operational Semantics
Imagine one tuple-variable for each relation in the FROM clause.
* These tuple-variables visit each combination of tuples, one from each relation.
If the tuple-variables are pointing to tuples that satisfy the WHERE clause,
drinker
bar
Sally
Joe’s
drinker
beer
Sally
Bud
Check these are equal
tv1 2 2
tv2
SELECT beer
FROM Likes, Frequents
WHERE bar = ’Joe’’s Bar’ AND
Frequents.drinker = Likes.drinker; 1
Frequents
check for Joe
Likes

80. Operational Semantics
Imagine one tuple-variable for each relation in the FROM clause.
* These tuple-variables visit each combination of tuples, one from each relation.
If the tuple-variables are pointing to tuples that satisfy the WHERE clause, send these tuples to the SELECT clause.
to output
drinker
bar
Sally
Joe’s
drinker
beer
Sally
Bud
Check these are equal 3
tv1 2 2
tv2
SELECT beer
FROM Likes, Frequents
WHERE bar = ’Joe’’s Bar’ AND
Frequents.drinker = Likes.drinker; 1
Frequents
check for Joe
Likes

81. Explicit Tuple-Variables

82. Explicit Tuple-Variables
Sometimes, a query needs to use two copies of the same relation.

83. Explicit Tuple-Variables
Sometimes, a query needs to use two copies of the same relation.
Distinguish copies by following the relation name by the name of a tuple-variable, in the FROM clause.

84. Explicit Tuple-Variables
Sometimes, a query needs to use two copies of the same relation.
Distinguish copies by following the relation name by the name of a tuple-variable, in the FROM clause.
It’s always an option to rename relations this way, even when not essential.

85. Explicit Tuple-Variables
Sometimes, a query needs to use two copies of the same relation.
Distinguish copies by following the relation name by the name of a tuple-variable, in the FROM clause.
It’s always an option to rename relations this way, even when not essential.
Example. From Beers(name, manf), find all pairs of beers by the same manufacturer.

86. Explicit Tuple-Variables
Sometimes, a query needs to use two copies of the same relation.
Distinguish copies by following the relation name by the name of a tuple-variable, in the FROM clause.
It’s always an option to rename relations this way, even when not essential.
Example. From Beers(name, manf), find all pairs of beers by the same manufacturer.
Do not produce pairs like (Bud, Bud).
Do not produce the same pair twice like (Bud, Miller) and (Miller, Bud)

87. Explicit Tuple-Variables
Sometimes, a query needs to use two copies of the same relation.
Distinguish copies by following the relation name by the name of a tuple-variable, in the FROM clause.
It’s always an option to rename relations this way, even when not essential.
Example. From Beers(name, manf), find all pairs of beers by the same manufacturer.
Do not produce pairs like (Bud, Bud).
Do not produce the same pair twice like (Bud, Miller) and (Miller, Bud)
SELECT b1.name, b2.name
FROM Beers b1, Beers b2
WHERE b1.manf = b2.manf AND b1.name < b2.name;