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Chapter 18
JDBC
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Background: databases
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Background: databases
• Types of databases
--Hierarchical
--Relational
--Object Relational
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Background: databases
• Types of databases
--Hierarchical-the first database, invented by IBM,
called IMS.
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Background: databases
• Types of databases
--Relational-the most common, invented by IBM but
first marketed by Oracle. Examples:
Oracle
DB2
Sybase
Access*
* Toy Database
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Background: databases
• Types of databases
--Object Relational-uncommon, attempts to place
objects in the database.
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Relational Databases
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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 Databases
The primary key attribute(s) cannot contain a null
value
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Relational Databases: Entity Integrity
• Here is a sample
table, USER
ID
LastName
Primary Key—must uniquely
identify a row. No nulls allowed.
FirstName
Age
1
Jones
Sam
32
2
Jones
Angela
27
3
Smith
Ann
22
4
Doe
Jack
44
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Relational Databases: Entity Integrity
• Here is another sample
table, CODE
Primary Key—must uniquely
identify a row. No nulls.
Code
Message
A
Bill Paid
B
Bill Overdue
C
Account written off
D
Account closed
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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 Databases
If 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 Databases
It is okay for a foreign key column to contain nulls.
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Relational Databases: Referential Integrity
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”.
ID
ID LastName
LastName
1 1 Jones
Jones
2 2 Jones
Jones
33
44
Smith
Smith
Doe
Doe
FirstName
FirstName
Sam
Sam
Age
Age
32
32
Code
Angela
Angela
Ann
Ann
27
27
22
22
B
Jack
Jack
44
44
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SQL Basics
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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 questions
updates—for making changes
insert—for adding new data
DDL—for creating tables
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SQL Basics: Structure of a SQL Statement
• Queries: SELECT statements
SELECT columns FROM table;
Or if we wish not to select all columns:
SELECT columns
FROM table
WHERE expression
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SQL Basics: Structure of a SQL Statement
• Queries: SELECT statements
SELECT FirstName, LastName
FROM USER
WHERE ID = 2;
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SQL Basics: Structure of a SQL Statement
• Updates: UPDATE statements
UPDATE table SET column = value;
Example:
UPDATE table
SET LastName = ‘Jones’
WHERE ID = 2;
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SQL Basics: Structure of a SQL Statement
• Insert: INSERT statements
INSERT INTO table VALUES(values);
Example:
INSERT INTO USER
VALUES( ‘6’, ‘Anderson’, ‘Joe’, 44, ‘A’)
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JDBC Basics:
Connection
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JDBC Basics: Connection
Three-tier Architecture for Data Access
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JDBC Basics: Connection
Three-tier Architecture for Data Access
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JDBC Basics: Connection
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Architecture for Microsoft Access ODBC
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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.
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JDBC Basics: Connection
• 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.
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JDBC Basics: Connection
• 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:
username
password
url
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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.
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JDBC Basics: Connection
• 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 close the connection you opened.
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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.
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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”;
Connection con = DriverManager.getConnection( url, user, pass );
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery( query );
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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 = ?”;
Connection con = DriverManager.getConnection( url, user, pass );
PreparedStatement pstmt = con.prepareStatement(
pstmt.setString( 1, 494 );
query
);
This line substitutes 494 for the
first question mark in the query.
ResultSet rs = pstmt.executeQuery();
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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 }”;
Connection con = DriverManager.getConnection( url, user, pass );
CallableStatement cstmt = con.prepareCall( call );
ResultSet rs = cstmt.executeQuery();
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JDBC Basics: ResultSet
• The ResultSet object receives the results of the query.
String query = “SELECT COL1, COL2 FROM MYTABLE WHERE ID = 2”;
Connection con = DriverManager.getConnection( url, user, pass );
Statement stmt = con.createStatement();
ResultSet rs = stmt.executeQuery( query );
next() returns true while there are results
while( rs.next() )
{
String myCol1 = rs.getString( “COL1” );
String myCol2 = rs.getString( “COL2” );
}
These correspond to
columns in the original
query.
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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 close your
ResultSet!
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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();
You must close these
three items, in the reverse
order that you opened
them!
}
catch( SQLException sql )
{
System.out.println( “Uh oh…” );
}
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DataSource
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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 pooling, where new
connections are not thrown away but are instead set aside for
the next time someone needs a connection.
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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 names that associate with resources
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DataSource: Need to Lookup in JNDI
• First we need to create an InitialContext so we can
lookup that DataSource
This is the name I
Context ctx = new InitialContext();
String dbJNDI = "java:comp/env/OracleJDBC";
assigned to the
DataSource when I
created it. Here, I’m just
looking it up under the
name I stored it
DataSource ds = (DataSource) ctx.lookup( dbJNDI );
Connection con = ds.getConnection();
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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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