--The SQL Query Language DML--1 LIKE LIKE allows to select character strings which have some element in common by using wild cards: Wild cards: “%”

--The SQL Query Language DML-- 1

LIKE

LIKE allows to select character strings which have some element in common by using wild cards:

Wild cards: “%” means “zero, one or more characters” “_” means “one single character”


SELECT: Queries Over Several Tables

Which films name ends in “100”?

SELECT TitleFROM Film

WHERE Title LIKE ‘%100’

Which films name contains the character a, b and c in the first, third and fifth positions?

SELECT TitleFROM Film

WHERE Title LIKE ‘a_b_c%’


Correlated Subqueries

Subquery must be re-evaluated for each tuple in outer SELECT

Table variable is used Find the customers who live at more than one address

(assume the key of Customer is (CustomerID, Address)).

SELECT Name

FROM Customer C

WHERE CustomerID IN

(SELECT D.CustomerID

FROM Customer D

WHERE C.Address <> D.Address)


Multiple Attribute Producing Subquery

The subquery produces a table with several columns EXISTS

true if subquery produced table has a tuple

NOT EXISTS true if subquery produced table is empty


EXISTS

List the customers who live in Dublin and have a film reserved.

SELECT NameFROM CustomerWHERE City = ’Dublin’

AND EXISTS ( SELECT * FROM Reserved WHERE Reserved.CustomerID =

Customer.CustomerID)


EXISTS, cont.

Often, EXISTS can be replaced with another correlation name.

List the customers who live in Dublin and have a film reserved.

SELECT NameFROM Customer, ReservedWHERE City = ’Dublin’

AND Customer.CustomerID = Reserved.CustomerID


NOT EXISTS

List the customers who live in Dublin but have no films reserved.

SELECT NameFROM CustomerWHERE City = ’Dublin’

AND NOT EXISTS(SELECT * FROM Reserved

WHERE Reserved.CustomerID = Customer.CustomerID)


Outline - The SELECT statement

Single table Projection Selection

Multiple tables Cartesian product and join Set operations Subqueries

Optional clauses Ordering results Computing aggregates on groups

Additional joins


ORDER BY

Can sort the result of a select, using ORDER BY. Who has reserved a film?

SELECT NameFROM Customer, ReservedWHERE Customer.CustomerID = Reserved.CustomerIDORDER BY Name

Can also sort in descending order, via DESC (ASC is the default).

SELECT NameFROM Customer, ReservedWHERE Customer.CustomerID = Reserved.CustomerIDORDER BY Name DESCOnly columns in the select list can be used for the ordering.


Select in the From Clause

The table in a from clause can itself be a select statement.

List the customers who have reserved an expensive film.

SELECT CustomerIDFROM Reserved,(SELECT *

FROM FilmWHERE RentalPrice > 4) AS Expensive

WHERE Reserved.FilmID = Expensive.FilmID

A correlation name is required in such cases.


Aggregates

Aggregates operate on the set of values of a column of a table, and return a single value. SUM: sum of values AVG: average value MAX: maximum value MIN: minimum value COUNT: number of values

How many reservations are there?

SELECT COUNT (*)FROM Reserved


Aggregates, cont.

What is the total rental price of all films?

SELECT SUM(RentalPrice)FROM Film

Eliminate duplicates before computing aggregate with DISTINCTIn how many cities do customers reside?

SELECT COUNT(DISTINCT City)FROM Customer


Aggregates, cont.

What is the average rental price of reserved films?

SELECT AVG(RentalPrice)FROM Reserved, FilmWHERE Reserved.FilmID = Film.FilmID

Find the film(s) with the highest rental price.

SELECT TitleFROM FilmWHERE RentalPrice IN (SELECT MAX(RentalPrice)

FROM Film)


GROUP BY

The table can be partitioned into several groups specifying

GROUP BY <list of columns>

Each group has the same attribute values for the indicated columns.

Name Age

Pam 4

Pat 1

Joe 3

Joe 2

Pat 1

Pam 4

Joe 3

Joe 2

Groups

GROUP BY Name


GROUP BY, cont.

Aggregate is applied to each group. What is the oldest age for each name?

SELECT Name, MAX(Age)FROM Person

GROUP BY Name

Name Age

Pam 4

Pat 1

Joe 3

Joe 2

Groups

Pam 4

Pat 1

Joe 3

Joe 2

MAX is 1

MAX is 4

MAX is 3

Pam 4

Pat 1

Joe 3

Name Max(Age)


GROUP BY, cont.

What is the average rental price of reserved films, by kind?

SELECT Kind, AVG(RentalPrice)FROM Reserved, FilmWHERE Reserved.FilmID = Film.FilmIDGROUP BY Kind


HAVING

Individual groups can be eliminated by using HAVING.

List the names and average age per name, as long as the average is more than 2.

Name Age

Pam 4

Pat 1

Joe 3

Joe 2

Groups

Pam 4

Pat 1

Joe 3

Joe 2

AVG is 1

AVG is 4

AVG is 2.5

Pam 4.0

Joe 2.5

Name AVG(Age)

SELECT Name, AVG(Age)FROM Person GROUP BY Name

HAVING AVG(Age) > 2


HAVING, cont.

Columns in HAVING clause must appear in the GROUP BY (or be contained within an aggregate).

List the customers whose average rental price for reserved films is greater than $3.

SELECT NameFROM Customer, Reserved, FilmWHERE Customer.CustomerID = Reserved.CustomerID

AND Reserved.FilmID = Film.FilmIDGROUP BY Name

HAVING AVG(RentalPrice) > 3

Note, RentalPrice appears within an aggregate


Summary

A query in SQL can consist of up to six clauses, but only the first two, SELECT and FROM, are mandatory.

The clauses are specified in the following order. SELECT column(s) FROM table list WHERE condition GROUP BY grouping column(s) HAVING group condition ORDER BY sort list


Summary, cont.

The query is evaluated in a different order. The tables in the from clause are combined using Cartesian

products. The where predicate is then applied. The resulting tuples are grouped according to the group by

clause. The having predicate is applied to each group, possibly

eliminating some groups. The aggregate(s) are applied to each remaining group. The select clause is performed last (followed by order clause).


Order of Clause Evaluation

1. FROM

2. WHERE – optional

3. GROUP BY - optional, defaults to 1 group

4. HAVING – optional

5. SELECT - (generalized) projection

6. ORDER BY - optional

Documents

--The SQL Query Language DML--1 LIKE LIKE allows to select character strings which have some element in common by using wild cards: Wild cards: “%”