1. More on Structured Query Language
Indra Budi

2. Join Expression
Permitting user to specify a table resulting from join operation in FROM clause of a query
Types of join:
Cross join (cross product)
Natural join
Theta join
Outer join 2

3. Join Expression Cross join Natural join
Join two tables without any conditions
Similar to concept of Cartesian product in set concept
SQL syntax:
<relation> CROSS JOIN <relation>
Natural join
Requiring the two join attributes have the same attribute names
Change the attribute name if the attributes have different name
The resulting relation contains all attributes from the two tables, but only one join attribute is kept
<relation> NATURAL JOIN <relation> 3

4. Join Expression Theta join
Joining two related tables using join condition in FROM clause
The resulting relation contains all attributes from the two tables, including the attribute(s) for join condition
SQL Syntax:
<relation> JOIN <relation> ON <condition> 4

5. Join Expression Outer join
Similar to theta join, the difference:
Theta join only returns the tuples with a matching tuple (relation)
Outer join returns all tuples with or without matching tuple (relation)
Types: Left, right and full outer join
SQL Syntax:
<relation1> OUTER JOIN <relation2>
It is modified by:
Optional NATURAL in front of OUTER.
Optional ON <condition> after JOIN.
Optional LEFT, RIGHT, or FULL before OUTER.
LEFT = pad dangling tuples of relation1 only.
RIGHT = pad dangling tuples of relation2 only.
FULL = pad both; this choice is the default. 5

6. Example-Cross Join SELECT * FROM Beers CROSS JOIN Likes Likes Beers
name
manf
Beer 1
XYZ
Beer 2
ABC
Beer 3
name
manf
drinker
Beer
Beer 1
XYZ
Andika
Panca
Mahesa
Beer 3
Beer 2
ABC
Likes
drinker
beer
Andika
Beer 1
Panca
Mahesa
Beer 3 6

7. Example-Natural Join SELECT * FROM Beers NATURAL JOIN Likes Frequents
drinker
beer
Andika
Beer 1
Mahesa
Panca
Beer 3
Siti
Beer 2
SELECT * FROM Beers NATURAL JOIN Likes
drinker
beer
bar
Andika
Beer 1
Zanz
Panca
Beer 3
ABC
Frequents
drinker
bar
Tyas
ABC
Bambang
XYZ
Panca
Andika
Zanz 7

8. Example-Theta Join
Beers
name
manf
Beer 1
XYZ
Beer 2
ABC
Beer 3
SELECT * FROM Beers B JOIN Likes L ON B.name = L.beer
name
manf
drinker
beer
Beer 1
XYZ
Aryo
Lamo
Beer 3
ABC
Amir
Likes
drinker
beer
Aryo
Beer 1
Lamo
Amir
Beer 3 8

9. Example-Outer Join Likes Beers
SELECT * FROM Beers B LEFT OUTER JOIN Likes L ON B.name = L.beer
Beers
name
manf
drinker
beer
Beer 1
XYZ
Aryo
Amir
Beer 2
ABC
Beer 3
Fitria
name
manf
Beer 1
XYZ
Beer 2
ABC
Beer 3
Likes
SELECT * FROM Beers B RIGHT OUTER JOIN Likes L ON B.name = L.beer
drinker
beer
Aryo
Beer 1
Amir
Siti
Beer 3
Fitria
Beer 5
name
manf
drinker
beer
Beer 1
XYZ
Aryo
Amir
Beer 3
ABC
Siti
Fitria
Beer 5 9

10. Example-Outer Join Likes Beers
name
manf
Beer 1
XYZ
Beer 2
ABC
Beer 3
SELECT * FROM Beers B FULL OUTER JOIN Likes L ON B.name = L.beer
name
manf
drinker
beer
Beer 1
XYZ
Aryo
Amir
Beer 2
ABC
Beer 3
Siti
Fitria
Beer 5
Likes
drinker
beer
Aryo
Beer 1
Amir
Siti
Beer 3
Fitria
Beer 5 10

11. Aggregations
SUM, AVG, COUNT, MIN, and MAX can be applied to a column in a SELECT clause to produce that aggregation on the column.
Also, COUNT(*) counts the number of tuples. 11

12. Example: Aggregation
From Sells(bar, beer, price), find the average price of Bud:
SELECT AVG(price)
FROM Sells
WHERE beer = ‘Bud’;
From Sells(bar, beer, price), find the cheapest and the most expensive price of Bud:
SELECT MIN(price), MAX(price) 12

13. Eliminating Duplicates in an Aggregation
DISTINCT inside an aggregation causes duplicates to be eliminated before the aggregation.
Example: find the number of different prices charged for Bud:
SELECT COUNT(DISTINCT price)
FROM Sells
WHERE beer = ‘Bud’; 13

14. Example Sells SELECT AVG(price) as average_price FROM Sells
WHERE beer = ‘Beer 2’;
Sells
average_price
7.33
bar
beer
price
ABC
Beer 1 5
Beer 2 8
XYZ
Beer 3
FGH 7
WXY
SELECT AVG(DISTINCT price) as average_price
FROM Sells
WHERE beer = ‘Beer 2’;
average_price
7.5 14

15. NULL’s Ignored in Aggregation
NULL never contributes to a sum, average, or count, and can never be the minimum or maximum of a column.
But if there are no non-NULL values in a column (all values are NULL), then the result of the aggregation is NULL. 15

16. Example: Effect of NULL’s
SELECT count(*)
FROM Sells
WHERE beer = ‘Bud’;
The number of bars
that sell Bud.
bar
beer
price
ABC
Beer 1 5
Bud 8
XYZ
Beer 3
FGH 7
WXY
The result : 3
SELECT count(price)
FROM Sells
WHERE beer = ‘Bud’;
The number of bars
that sell Bud at a
known price (price not
NULL)
The result : 2 16

17. Grouping
We may follow a SELECT-FROM-WHERE expression by GROUP BY and a list of attributes.
The relation that results from the SELECT-FROM-WHERE is grouped according to the values of all those attributes, and any aggregation is applied only within each group. 17

18. Example: Grouping
From Sells(bar, beer, price), find the average price for each beer:
SELECT beer, AVG(price)
FROM Sells
GROUP BY beer;
From Sells(bar, beer, price), find the cheapest price for each beer:
SELECT beer, MIN(price) 18

19. Example: Grouping
From Sells(bar, beer, price) and Frequents(drinker, bar), find for each drinker the average price of Bud at the bars they frequent:
SELECT drinker, AVG(price)
FROM Frequents, Sells
WHERE beer = ‘Bud’ AND
Frequents.bar = Sells.bar
GROUP BY drinker;
Compute
drinker-bar-
price of Bud
tuples first,
then group
by drinker. 19

20. Restriction on SELECT Lists With Aggregation
If any aggregation is used, then each element of the SELECT list must be either:
Aggregated, or
An attribute on the GROUP BY list. 20

21. Illegal Query Example
You might think you could find the bar that sells Bud the cheapest by:
SELECT bar, MIN(price)
FROM Sells
WHERE beer = ‘Bud’;
But this query is illegal in SQL.
Why? Note bar is neither aggregated nor on the GROUP BY list. 21

22. HAVING Clauses HAVING vs WHERE
HAVING  filtering group
WHERE  filtering tuples
HAVING <condition> may follow a GROUP BY clause.
If so, the condition applies to each group, and groups not satisfying the condition are eliminated. 22

23. 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. 23

24. Example: HAVING
From Sells(bar, beer, price), find the beer name and cheapest price for that beer ranging from 4 to 10.
SELECT beer, MIN(price)
FROM Sells
GROUP BY beer
HAVING MIN(price) >= 4 AND MIN(price) <=10 24

25. Example: HAVING
From Sells(bar, beer, price) and Beers(name, manf), find the average price of those beers that are either served in at least three bars or are manufactured by Pete’s. 25

26. Solution SELECT beer, AVG(price) FROM Sells GROUP BY beer
Beer groups with at least
3 non-NULL bars and also
beer groups where the
manufacturer is Pete’s.
SELECT beer, AVG(price)
FROM Sells
GROUP BY beer
HAVING COUNT(bar) >= 3 OR
beer IN (SELECT name
FROM Beers
WHERE manf = ‘Pete’’s’);
Beers manu-
factured by
Pete’s. 26

27. Database Modifications
A modification command does not return a result as a query does, but it changes the database in some way.
There are three kinds of modifications:
Insert a tuple or tuples.
Delete a tuple or tuples.
Update the value(s) of an existing tuple or tuples. 27

28. 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’); 28

29. Specifying Attributes in INSERT
We may add to the relation name a list of attributes.
There are two reasons to do so:
We forget the standard order of attributes for the relation.
We don’t have values for all attributes, and we want the system to fill in missing components with NULL or a default value. 29

30. Example: Specifying Attributes
Another way to add the fact that Sally likes Bud to Likes(drinker, beer):
INSERT INTO Likes(beer, drinker)
VALUES(‘Bud’, ‘Sally’); 30

31. Inserting Many Tuples
We may insert the entire result of a query into a relation, using the form:
INSERT INTO <relation>
( <subquery> ); 31

32. Example: Insert a Subquery
Using Frequents(drinker, bar), 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

33. Solution INSERT INTO PotBuddies (SELECT d2.drinker
The other
drinker
Pairs of Drinker
tuples where the
first is for Sally,
the second is for
someone else,
and the bars are
the same.
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

34. Deletion To delete tuples satisfying a condition from some relation:
DELETE FROM <relation>
WHERE <condition>; 34

35. Example: Deletion
Delete from Likes(drinker, beer) the fact that Sally likes Bud:
DELETE FROM Likes
WHERE drinker = ‘Sally’ AND
beer = ‘Bud’; 35

36. Example: Delete all Tuples
Make the relation Likes empty:
DELETE FROM Likes;
Note no WHERE clause needed.
Do this command delete the Likes table from database ? 36

37. 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. 37

38. Updates To change certain attributes in certain tuples of a relation:
UPDATE <relation>
SET <list of attribute assignments>
WHERE <condition on tuples>; 38

39. Example: Update Change drinker Fred’s phone number to 555-1212:
UPDATE Drinkers
SET phone = ‘ ’
WHERE name = ‘Fred’; 39

40. Example: Update Several Tuples
Make $4 the maximum price for beer:
UPDATE Sells
SET price = 4.00
WHERE price > 4.00; 40

41. SQL Support for Views Views are Part of the SQL DDL
Abstracting from Conceptual to External Schema
View Hides the Details
One or More Tables in Conceptual Schema May be Combined (in Part) to Form a View
Don’t Include FKs and Other Internal Attributes
Typically, View is Formed by Join of Two or More Relations Utilizing FKs, PKs, etc.
As a Result - View is Independent
Once Formed - View Static/Unchangeable to Insulate User Applications from Conceptual Schema
Similar in Concept/Intent to “Public Interface” 41

42. SQL View Definition Features of Views Reasons for Views
View Represents a Restricted Portion (Rows, Columns) of a Relation - External Schema in SQL
View is Virtual Table View (Not Stored) and Must be Re-evaluated Every Time - Dynamic
Like Relation, a View Can Be Deleted at Any Time
Attributes Can Be Renamed in View
Reasons for Views
Security
Increasing Application-Data Independence
CREATE VIEW PQ(P#, SUMQTY)
AS SELECT P#, SUM(QTY)
FROM SP
GROUP BY P#; 42

43. View Definition in Ongoing Example
First View: Attribute Names are Inherited
CREATE VIEW WORKS_ON1 AS
SELECT FNAME, LNAME, PNAME, HOURS
FROM EMPLOYEE, PROJECT, WORKS_ON
WHERE SSN=ESSN AND PNO=PNUMBER ;
Second View: View attribute names are Aliased via a one-to-one Correspondence with the SELECT-clause
CREATE VIEW DEPT_INFO
(DEPT_NAME, NO_OF_EMPS, TOTAL_SAL) AS
SELECT DNAME, COUNT (*), SUM (SALARY)
FROM DEPARTMENT, EMPLOYEE
WHERE DNUMBER=DNO
GROUP BY DNAME ; 43

44. Queries on Views
Retrieve the Last Name and First Name of All Employees Who Work on 'ProjectX'. SELECT PNAME, FNAME, LNAME FROM WORKS_ON1 WHERE PNAME='ProjectX' ;
Without the View WORKS_ON1, this Query Specification Would Require Two Join Conditions
A View Can Be Defined to Simplify Frequently Occurring Queries
DBMS Keeps the View Up-to-date if the Base Tables on Which the View is Defined are Modified
Hence, the View is Realized at the Time we Specify a Query on the View 44

45. What is View Update Problem?
Retrieval over View Mirrors a Retrieval over Relation
However, Update over View may cause Problems!
In general, a View Update may Introduce Ambiguity when there is more than one way to Update Underlying Relations
Consider the view PQ Created Below:
Try to Change the Total Quantity SUMQTY of P1 in PQ from “30” to “40”
Why Does a view Update Problem occur?
CREATE VIEW PQ(P#, SUMQTY)
AS SELECT P#, SUM(QTY)
FROM SP
GROUP BY P#;
If we want to change the total quantity SUMQTY of a part P1 in PQ from “30” to “40”, a view update problem incurs, because this update over view PQ does not give enough information to enable us to map the view update to the base relations without ambiguity. 45