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Chapter 18

JDBC


Background: databases


• Types of databases

--Hierarchical

--Relational

--Object Relational




--Hierarchical-the first database, invented by IBM, called IMS.




--Relational-the most common, invented by IBM but first marketed by Oracle. Examples:

OracleDB2SybaseAccess*

* Toy Database




--Object Relational-uncommon, attempts to place objects in the database.



Relational Databases


Relational Databases: Entity Integrity

• Unit of the Table, smallest unit in a relational database• For a table to be useful, it must enforce Entity Integrity.

Entity Integrity—each row in a table can be located

by using its Primary Key.1st Law of Relational Databases

Each row in a table must have an attribute(s) that uniquely locates one row. Values in this attribute must be unique.

2nd Law of Relational DatabasesThe primary key attribute(s) cannot

contain a null value



• Here is a sample table, USER

ID LastName FirstName Age

1 Jones Sam 32

2 Jones Angela 27

3 Smith Ann 22

4 Doe Jack 44

Primary Key—must uniquely identify a row. No nulls allowed.



• Here is another sample table, CODE

Code Message

A Bill Paid

B Bill Overdue

C Account written off

D Account closed

Primary Key—must uniquely identify a row. No nulls.


Relational Databases: Referential Integrity

• A “foreign key” is when the primary key of one table is repeated in a second table as a non primary key.• Using foreign keys, or “referential integrity” allows us to link tables.

3rd Law of Relational DatabasesIf you link two tables with a foreign key,

any values present in the foreign-key attribute column must link back to existing primary-key values.

4th Law of Relational DatabasesIt is okay for a foreign key column to

contain nulls.


ID LastName FirstName Age

1 Jones Sam 32

2 Jones Angela 27

3 Smith Ann 22

4 Doe Jack 44

Relational Databases: Referential Integrity

ID LastName FirstName Age Code

1 Jones Sam 32 A

2 Jones Angela 27 B

3 Smith Ann 22 A

4 Doe Jack 44

This is the primary key for another table. This column can contain nulls. However, any values present must exist in the table that is referred to. This is a “foreign key”.


SQL Basics


SQL Basics: Structure of a SQL Statement

• When accessing a relational database, you must use the “Structured Query Language” (SQL)

• Several types of SQL:queries—for asking questionsupdates—for making changesinsert—for adding new dataDDL—for creating tables



• Queries: SELECT statements

SELECT columns FROM table;

Or if we wish not to select all columns:

SELECT columns FROM table WHERE expression



• Queries: SELECT statements

SELECT FirstName, LastNameFROM USERWHERE ID = 2;



• Updates: UPDATE statements

UPDATE table SET column = value;

Example:

UPDATE table SET LastName = ‘Jones’ WHERE ID = 2;



• Insert: INSERT statements

INSERT INTO table VALUES(values);

Example:

INSERT INTO USERVALUES( ‘6’, ‘Anderson’, ‘Joe’, 44, ‘A’)


JDBC Basics: Connection



• The first step toward connecting to a database is getting a database connection.

• Before you can get a connection, you need a database driver.

• The driver makes the connection between a particular database and our Java program.

• These drivers are individual to each vendor’s database.



• To make sure your driver is available, you use the following:

Class.forName( “sun.jdbc.odbc.JdbcOdbcDriver” );

• The above statement will merely ensure that the Java class containing the driver is available to our program.



• The statement below results in a connection to the database.

import java.sql.Connection;…

Connection con = DriverManager.getConnection( url, user, pass );

• To create a connection using this method, it is necessary to pass three arguments to the method:

usernamepasswordurl


JDBC Basics: url

• The Url is a special string of characters that finds the database.• Here is a sample Url:

url = jdbc:oracle:thin:@myhostname:1521:OurDB

jdbc:oracle:thin:@ —This is database specific

myhostname —This is the name of the host where the database is located.

1521 —This is the port on the host where the database is listening.

OurDB —This is the name of the database.



• The DriverManager is convenient but not scalable.

import java.sql.Connection;…Connection con = DriverManager.getConnection( url, user, pass );

• Once you have opened a connection to the database, you must realize this is a resource.

• You must You must closeclose the connection you opened. the connection you opened.


JDBC Basics: Statements

• After you have a connection, you need to create a statement. • There are three alternatives, each with plusses and minuses.

Statement—used for a query that will be executed once.

PreparedStatement—used for a query that will be executed multiple times

CallableStatement—used for a query that executes a stored procedure.


JDBC Basics: Statement

• The Statement object is the easiest to work with.• The Statement object is the least efficient.

String query = “SELECT * FROM MYTABLE WHERE ID = 2”;


Statement stmt = con.createStatement();

ResultSet rs = stmt.executeQuery( query );


JDBC Basics: PreparedStatement

• The PreparedStatement object requires more work.• The PreparedStatement object is the most efficient.• The query contains a question mark that is replaced.

String query = “SELECT * FROM MYTABLE WHERE ID = ?”;


PreparedStatement pstmt = con.prepareStatement( query );pstmt.setString( 1, 494 );

ResultSet rs = pstmt.executeQuery();

This line substitutes 494 for the first question mark in the query.


JDBC Basics: CallableStatement

• The CallableStatement object is only appropriate for calling a stored procedure.• The syntax of how you call the stored procedure is database specific.

String call = “{ call myProcdure }”;


CallableStatement cstmt = con.prepareCall( call );

ResultSet rs = cstmt.executeQuery();


JDBC Basics: ResultSet

• The ResultSet object receives the results of the query.

String query = “SELECT COL1, COL2 FROM MYTABLE WHERE ID = 2”;


Statement stmt = con.createStatement();

ResultSet rs = stmt.executeQuery( query );

while( rs.next() ){ String myCol1 = rs.getString( “COL1” ); String myCol2 = rs.getString( “COL2” );}

next() returns true while there are results

These correspond to columns in the original

query.


JDBC Basics: ResultSet

• No matter which kind of statement you choose, the ResultSet object is used the same way.• As with the Connection object, you must closeclose your ResultSet!


try{ String output = null; String query = “SELECT username from MYTABLE where pass=‘foo’ ”; Connection con = DriverManager.getConnection( url, us, pass); Statement stmt = con.createStatement(); ResultSet rs = stmt.executeQuery( query );

while( rs.next() ) { output = rs.getString( “username” ); }

rs.close(); stmt.close(); con.close();}catch( SQLException sql ){ System.out.println( “Uh oh…” );}

You must close these three items,

in the reverse order that you opened them!


DataSource


DataSource

• As I said, the DriverManager is not the best choice for a production system. It doesn’t scale well.

• A better alternative is using a DataSource.

• A DataSource offers connection poolingconnection pooling, where new connections are not thrown away but are instead set aside for the next time someone needs a connection.


DataSource: Need to Lookup in JNDI

• To use a DataSource, it is necessary to perform a lookup of the resource in something called JNDI[ JNDI = Java Naming and Directory Interface ]

• JNDI stores a list of namesnames that associate with resourcesresources


DataSource: Need to Lookup in JNDI

• First we need to create an InitialContext so we can lookup that DataSource

Context ctx = new InitialContext();

String dbJNDI = "java:comp/env/OracleJDBC";

DataSource ds = (DataSource) ctx.lookup( dbJNDI );

Connection con = ds.getConnection();

This is the name I assigned to the

DataSource when I created it. Here, I’m just looking it

up under the name I stored it


DataSource:Complexity of setup

• Using a DataSource is very valuable because it allows connection pooling.

• The downside of using a DataSource is the complexity of its setup. Also, each Application Server vendor has its own unique setup. You will need to learn these*.

* Please refer to “Setting up a DataSource in WebSphere Application Server 5.1”

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