1

Rewriting Intersect Queries Using In

SELECT S.sid

FROM Sailors S, Boats B, Reserves R

WHERE S.sid = R.sid and R.bid = B.bid and B.color = ‘red’

INTERSECT

SELECT S.sid

FROM Sailors S, Boats B, Reserves R

WHERE S.sid = R.sid and R.bid = B.bid and B.color = ‘green’;

2

Rewriting Intersect Queries Using In

SELECT S.sid

FROM Sailors S, Boats B, Reserves R

WHERE S.sid = R.sid and R.bid = B.bid and B.color = ‘red’ and S.sid IN

(SELECT S2.sid

FROM Sailors S2, Boats B2, Reserves R2

WHERE S.sid = R.sid and R.bid = B.bid and B.color = ‘green’);

3

Division

• Consider: A(X,Y) and B(Y).

Then A/B =

• In general, we require that the set of fields in B be contained in those of A.

A}, B)y( |x{ yx

4

Suppliers from A who supply All Parts from B

sno pno

S1

S1

S1

S1

S2

S2

S3

S4

S4

P1

P2

P3

P4

P1

P2

P2

P2

P4

A

P2

pno

B1

P2

P4

pno

B2

P1

P2

P4

pno

B3

S1

S4

sno

A/B2

S1

sno

A/B3

S1

S2

S3

S4

sno

A/B1

5

Sailors who Reserved all Boats

• To find the Sailors who reserved all boats:

(sid,bid

Reserves)/(bid

Boats)

• Division can be expressed using other relational algebra operators.

6

Division in SQL (1)

SELECT S.sname

FROM Sailors S

WHERE NOT EXISTS(SELECT B.bid

FROM Boats B

WHERE NOT EXISTS(SELECT R.bid

FROM Reserves R

WHERE R.bid=B.bid

and

R.sid=S.sid

7

Division in SQL (2)

SELECT S.sname

FROM Sailors S

WHERE NOT EXISTS((SELECT B.bid

FROM Boats B)

EXCEPT

(SELECT R.bid

FROM Reserves R

WHERE R.sid = S.sid));

8

Aggregation

9

Aggregate Operators

• The aggregate operators available in SQL are:– COUNT(*)– COUNT([DISTINCT] A)– SUM([DISTINCT] A)– AVG([DISTINCT] A)– MAX(A)– MIN(A)

10

Some Examples

SELECT COUNT(*)

FROM Sailors S

SELECT AVG(S.age)

FROM Sailors S

WHERE S.rating=10

11

Find name and age of oldest Sailor

SELECT S.sname, MAX(S.age)

FROM Sailors S

SELECT S.sname, S.age

FROM Sailors S

WHERE S.age =

(SELECT MAX(S2.age)

FROM Sailors S2)

Wrong!!

Right!!

12

Find Average Age for each Rating

• So far, aggregation has been applied to all tuples that passed the WHERE clause test.

• How can we apply aggregation to groups of tuples?

13

Basic SQL Query

SELECT [Distinct] target-list

FROM relation-list

WHERE condition

GROUP BY grouping-list

HAVING group-condition;

•target-list: Fields appearing in grouping-list and aggregation operators

•group-condition: Can only constrain attributes appearing in grouping-list

14

Evaluation

1. Compute cross product of relations in FROM

2. Tuples failing WHERE are thrown away

3. Tuples are partitioned into groups by values of grouping-list attributes

4. The group-condition is applied to eliminate groups

5. One answer in generated for each group

15

Find Average Age for each Rating

SELECT S.rating, AVG(S.age)

FROM Sailors S

GROUP BY S.rating

16

Find the number of Reservations of Each Red

Boat

SELECT B.bid, COUNT(*) as count

FROM Boats B, Reserves R

WHERE R.bid=B.bid and B.color=‘red’

GROUP BY B.bid

What would happen if we put the condition about the color in the HAVING clause?

17

Age of Youngest Sailor that is over 18, for each Rating with

at least 2 such Sailors

SELECT S.rating, MIN(S.age)

FROM Sailors S

WHERE S.age >= 18

GROUP BY S.rating

HAVING COUNT(*) > 1

18

Age of Youngest Sailor that is over 18, for each Rating with at least 2 Sailors (of any Age)

SELECT S.rating, MIN(S.age)

FROM Sailors S

WHERE S.age >= 18

GROUP BY S.rating

HAVING 1 < (SELECT COUNT(*)

FROM Sailors S2

WHERE S.rating =

S2.rating)

19

More Options

20

Sorting Results

• Results can be sorted using the ORDER BY clause

• Here are sailors who ordered boat 103, ordered by their names:

SELECT S.sname

FROM Sailors S, Reserves R

WHERE S.sid = R.sid and R.bid = 103

ORDER BY S.sname

21

Outer Join

• How can we find sailor names with their reservation numbers if we don’t want to lose information about sailors who did not reserve any boats??

SELECT S.sname, R.bid

FROM Sailors S, Reserves R

WHERE S.sid = R.sid(+)

• The (+) must follow column that we allow to be null.

22

Views

• A View is a query that looks like a table and can be used as a table.

CREATE OR REPLACE VIEW SailorsBoats

SELECT S.sname, B.bname, B.color

FROM Sailors S, Reserves R, Boats B

WHERE S.sid = R.sid and R.bid = B.bid;

SELECT sname

FROM SailorBoats

WHERE color = ‘red’;

23

Views For Restricting Access

• Suppose that we have a table:Grades(Login, Exercise, Grade)

• We would like a user to only be able to see his own grades. We create the following view and grant privileges to query the view (not the underlying table)

CREATE OR REPLACE VIEW UserGrades

SELECT *

FROM Grades

WHERE Login = User;

Pseudo-column which is equal to the user name.

24

Views for Complex Queries

• Find those ratings for which the average age is the minimum over all ratings

CREATE OR REPLACE VIEW AvgAgeRatings

SELECT S.rating, AVG(S.age) as avgage

FROM Sailors S

WHERE S.rating;

SELECT rating, avgage

FROM AveAgeRatings

WHERE avgage = (SELECT MIN(avgage)

FROM AveAgeRatings)

25

Sub-queries in FROM, instead of Views

SELECT Temp.rating, Temp.avgage

FROM (SELECT S.rating, AVG(S.age) as avgage

FROM Sailors S

GROUP BY S.rating) as Temp

WHERE Temp.avgage = (SELECT MIN(Temp.avgage)

FROM Temp)

26

Delete and Update

27

Deleting Tuples

• The basic form of a delete statement is:

DELETE FROM TableName

WHERE Condition;

28

Examples

DELETE FROM Sailors

WHERE rating < 3;

DELETE FROM Sailors S1

WHERE S1.rating = (SELECT MIN(S2.rating) FROM Sailors S2)

Delete Sailors with rating less than 3:

Delete Sailors with the minimum rating:

29

Updating Tuples

• The basic form of an update statement is:

UPDATE TableName

SET Column1 = Value1, …

ColumnN = ValueN

WHERE Condition;

30

Example

UPDATE Boats

SET color = ‘red’,

bname = ‘Voyager’

WHERE bid = 13;

Update boat 13: color to red and name to voyager

31

Another Example

UPDATE Sailors

SET rating = (SELECT MAX(rating)

FROM Sailors)

WHERE sname = ‘Rusty’;

Update rating of Rusty to be the maximum rating of any sailor

Note: When updating with a subquery, the subquery must return one value only!