Practical Session 13

Structured Data BasesStructured Query Language

Exam QuestionsFactory Method PatternAbstract Factory Pattern

Relational Databases• A relational database is a collection of data items organized as a set of formally

described tables from which data can be accessed easily• A relational database is created using the relational model:

– Data Definition Language• Used to build and destroy databases, create, and drop tables

– Data Manipulation Language• Used to manipulate data in databases, insert, delete, and retrieve

• We will use SQL (Structured Query Language) data definition and query language.

• Each database consists of a number of tables and each table has its own primary key.

• Relational:– Because you may have relations between the different tables.

Very good tutorial at: http://www.w3schools.com/sql/default.asp

Table

- Table name: TEACHING_ASSISTANTS- Column name: Id, Name, Office Hours- Column type: INT, VARCHAR(20), VARCHAR(20)- Each table has primary key, must be unique and non-null: example: id- Each line in table is called a record- You may have foreign key columns, which is a primary key in other table, to

denote relationship between two different tables

ID Name Office hours

1 Achiya Tue 10:00-12:00

2 Boaz Sun 16:00-18:00

3 Jumana Sun 16:00-18:00

5 Majeed Sun 16:00-18:00

4 Ramzi Thu 12:00-14:00

6 Yehonatan Tue 12:00-14:00

ANSI SQL Data Types• Character strings

– CHARACTER(n) or CHAR(n) — fixed-width n-character string, padded with spaces as needed– CHARACTER VARYING(n) or VARCHAR(n) — variable-width string with a maximum size of n characters– NATIONAL CHARACTER(n) or NCHAR(n) — fixed width string supporting an international character set– NATIONAL CHARACTER VARYING(n) or NVARCHAR(n) — variable-width NCHAR string

• Bit arrays– BIT(n) — an array of n bits– BIT VARYING(n) — an array of up to n bits

• Numbers– INTEGER and SMALLINT– FLOAT, REAL and DOUBLE PRECISION– NUMERIC(precision, scale) or DECIMAL(precision, scale)

• Date and time– DATE — for date values (e.g., 2011-05-03)– TIME — for time values (e.g., 15:51:36). – TIME WITH TIME ZONE or TIMETZ — the same as TIME, but including details about the time zone in question.– TIMESTAMP — This is a DATE and a TIME put together in one variable (e.g., 2011-05-03 15:51:36).– TIMESTAMP WITH TIME ZONE or TIMESTAMPTZ — the same as TIMESTAMP, but including details about the time zone in

question.

Creating/Deleting a tableCREATE TABLE STORE(Id integer PRIMARY KEY,Name varchar(30) NOT NULL,Type varchar(30));

- SQL statements need to end with a semicolon to separate between different statements.

Primary Key

• A primary key is used to uniquely identify each row in a table. • A primary key can consist of one or more fields on a table.• When multiple fields are used as a primary key, they are called

a composite key.• Primary key is inheritably unique.

Foreign Key

• A foreign key is a field(s) that point to the primary key of another table.

• The purpose of the foreign key is to ensure referential integrity of the data.– Only values that are in the database are permitted.

Code

Example

Column Type Constraints

ID int Primary Key

Name VARCHAR(50)

Office hours VARCHAR(9)

Column Type Constraints

TA int Foreign Key

Group int Primary Key

Location VARCHAR(50)

Time VARCHAR(9)

TEACHING_ASSISTANTS

PRACTICAL_SESSIONS

Effect: PRACTICAL_SESSIONS table cannot contain information on a TA that is not in the Teaching Assistant table.

SQL Commands

PRACTICAL_SESSIONS

TA Group Location Time

3 11 90-234 Sun 14-16

3 12 34-205 Sun 18-20

4 13 90-125 Thu 14-16

6 21 28-145 Thu 14-16

1 22 28-107 Thu 14-16

2 23 72-213 Thu 14-16

5 31 90-145 Thu 14-16

6 32 90-127 Thu 14-16

2 33 90-134 Thu 14-16

1 41 90-235 Thu 14-16

5 42 90-235 Thu 14-16

ID Name OfficeHours

1 Achiya Tue 10:00-12:00

2 Boaz Sun 16:00-18:00

3 Jumana Sun 16:00-18:00

5 Majeed Sun 16:00-18:00

4 Ramzi Thu 12:00-14:00

6 Yehonatan Tue 12:00-14:00

TEACHING_ASSISTANTS

Insert/Delete/Update [record]

Select

• The most common operation in SQL is the query, which is performed with the declarative SELECT statement.

• SELECT retrieves data from one or more tables, or expressions. • Standard SELECT statements have no persistent effects on the

database. • Some non-standard implementations of SELECT can have

persistent effects, such as the SELECT INTO syntax that exists in some databases.

Select• A query includes a list of columns to be included in the final result immediately following the SELECT keyword. • An asterisk ("*") can be used to specify that the query should return all columns of the queried tables. • SELECT is the most complex statement in SQL, with optional keywords and clauses that include:

– FROM:• The FROM clause which indicates the table(s) from which data is to be retrieved.

– WHERE:• The WHERE clause includes a comparison predicate, which restricts the rows returned by the query. • The WHERE clause eliminates all rows from the result set for which the comparison predicate does not evaluate to

True.– GROUP BY:

• The GROUP BY clause is used to project rows having common values into a smaller set of rows. • GROUP BY is often used in conjunction with SQL aggregation functions or to eliminate duplicate rows from a result

set. The WHERE clause is applied before the GROUP BY clause.– HAVING:

• The HAVING clause includes a predicate used to filter rows resulting from the GROUP BY clause. • Because it acts on the results of the GROUP BY clause, aggregation functions can be used in the HAVING clause

predicate.– ORDER BY:

• The ORDER BY clause identifies which columns are used to sort the resulting data, and in which direction they should be sorted (options are: ASC (default) ascending or DESC descending).

• Without an ORDER BY clause, the order of rows returned by an SQL query is undefined.

SQL Aggregation Functions

• SQL aggregate functions return a single value, calculated from values in a column.– AVG() - Returns the average value– COUNT() - Returns the number of rows– FIRST() - Returns the first value– LAST() - Returns the last value– MAX() - Returns the largest value– MIN() - Returns the smallest value– SUM() - Returns the sum

FROM

• * returns all columns.• You may specifically choose columns you want, and their order

WHERE

• AND• OR• IS (IS NOT NULL)• IN• BETWEEN• LIKE• http://en.wikipedia.org/wiki/Where_%28SQL%29

HAVING/GROUP BY

HAVING/GROUP BY

ORDER BY

• If not specified, default order is ascending.

JOIN

• The JOIN keyword is used in an SQL statement to query data from two or more tables, based on a relationship between certain columns in these tables.

• Tables in a database are often related to each other with keys.• Different SQL JOINs:– INNER JOIN/JOIN: Return rows when there is at least one match in both tables.– LEFT JOIN: Return all rows from the left table, even if there are no matches in

the right table.– RIGHT JOIN: Return all rows from the right table, even if there are no matches in

the left table.– OUTER JOIN/FULL JOIN: Return rows when there is a match in one of the tables.

Examples: http://www.w3schools.com/sql/sql_join.asp

Exam Questions

• Write a model that holds the following information:– Movie: Name, publish year, origin country, director name– Director: Name, list of movies– Actor: Name, list of roles in movies

Notes• 1:N relations are encoded by a single foreign key in the table that has the N end:

– Movies <N : 1 > Director (In general a movie has only one director)• N:N relations are encoded by a cross-table; two foreign keys to the related

tables with additional information that characterizes the relation if needed– Movies <N : N > Actors (In general, actors play in many movies, and movies have many

actors)– In our case, the role played by the actor in the movie is data that belongs to the cross

table.• If you assume that a movie can have several directors, then you need a cross

table movies_directors as well.• If you assume that additional information on directors may not be added, or that

names of directors are not modified – then you may use the field directorName directly in the movies table.

Solution

Query on table

• Write an SQL query that returns: – Movie name, Director name, Actor, Role

Multiple Roles for Actor in Movie

• We wish to add support for multiple roles for each actor in a movie.

• Current implementation defines the primary key of actors_movies as (actorId, movieId).

• This means that we cannot have the same actor in the same movie more than once.

• Solution?– Add the field role to the primary key.– Primary key (actorId, movieId, role);

Query

• Write an SQL query that returns the roles of “Christoph Waltz” in movies that where directed by “Quentin Tarantino”

Answer?

• Inglorious Bastards• Django Unchained

Factory Method Pattern

• How it is done?– Making Constructors private/protected– Implementing a function which its sole purpose is creating desired

objects and returning them• create()• open()

• Can be done by using static creation functions.

Example

Abstract Factory Pattern

• Done by creating a class which its sole purpose is creating requested objects.

• The rest of the classes are not public, not part of the interface.• Any creation of objects need to be done by making an instance

of the Factory object and using its methods.

Example

• http://en.wikipedia.org/wiki/Abstract_factory_pattern#Java

When and why to use Factory over Constructors• Allows developers of the framework to change the constructors when needed,

without worrying about backward compatibility.– Frameworks which uses Constructors cannot change any constructor at all once the product is

released.– Releasing new versions of the product requires them to keep these constructors to allow

applications which already use their framework to continue working.– Solution? Factory Pattern.

• Allows creation of objects when it is unknown which type to create.– When deciding on object type relies on the state of the framework, using “new” is not

possible.– Users of framework cannot know internal state of the framework itself.– Using new operator already decides which object type to create.

• When the user does not really care about many of the constructor parameters.– Framework: Hiring Agency [agency]– User: Company wishes to hire a Java developer, with 3 years experience.

• Factory method: agency.hireDeveloper(“Java”, 3);• Constructor: new Worker(name, age, experience, skills, address, id);

– The “user” only cares about 2 things, however creating the object requires many more items.• If having a named method is the only way to make the creation self-explanatory,

consider factory pattern over regular constructors.– Constructor names must