16. jdbc&sql pack

8/11/2019 16. Jdbc&SQL Pack

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Accessing a Database Using the java.sqlpack


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JDBC is a standard Java class library, to query

and modify relational data Import java.sql.*in your Java code

JDBC is modeled after ODBC

Supports SQL92 syntax and types

Allows for vendor-specific extensions

Many Oracle extensions

What Is JDBC?


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Relationship Between JDBC Classes

ResultSet

ResultSetMetaData

DatabaseMetaData

Statement

Connection

DriverManager

1

6

4

2

3 5


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Registering a JDBC Driver

JDBC drivers must register themselves with theDriverManager

Drivers register themselves automatically when

they are loaded

try {

Class c = Class.forName(

"sun.jdbc.odbc.JdbcOdbcDriver");

}

catch (ClassNotFoundException e) {

e.printStackTrace();

}


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Example: Connecting to Oracle

The following code connects toan Oracle database

Uses the ODBC JDBC Bridge

driver

Connection conn;

try {

conn = DriverManager.getConnection(

"jdbc:odbc:dsn_name", theUser","thePassword");

}

catch (SQLException e) {}


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Obtaining Database Metadata

Connectioncan be used to get a

DatabaseMetaDataobject

This provides many methods to obtain metadata

for the database

Connection conn; try {

DatabaseMetaData dm = conn.getMetaData();

String s1 = dm.getURL();

String s2 = dm.getSQLKeywords();boolean b1 = dm.supportsTransactions();

boolean b2 = dm.supportsSelectForUpdate();

}



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Three interfaces are defined, providing the

following capabilities:

Execute queries and other

DML/DDL operationsExecute precompiled

statements

Call stored procedures

Statements

Statement

PreparedStatement

CallableStatement


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Statements and Queries

The Statementclass can be used as follows, to

execute a query

executeQuery()executes a SQL query, and

returns a ResultSet

try {

Statement stmt = conn.createStatement();ResultSet rset = stmt.executeQuery

("select ENAME, SAL from EMP");

}



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Processing Query Results

ResultSetholds a table of result data returned

by a SQL query

Support for cursors

Cursor starts at beginning of data set Use next()to move to next record

Retrieve data using getXXX()methods, mapping

results to equivalent Java types


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Processing Results: Example

try {

ResultSet rset = stmt.executeQuery(

"select ENAME, SAL from EMP");

while (rset.next()) {

String ename = rset.getString(1);BigDecimal sal = rset.getBigDecimal(2, 2);

// Can also access columns by name:

// String ename = rset.getString("ENAME");

// BigDecimal sal = rset.getBigDecimal("SAL");

}

}


Obt i i M t d t


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Obtaining ResultSetMetadata

ResultSetcan be used to get a

ResultSetMetaDataobject

Provides result set metadata

try {

ResultSet rset = ;ResultSetMetaData md = rset.getMetaData();

while (rset.next()) {

for (int i = 0; i < md.getColumnCount(); i++) {

String lbl = md.getColumnLabel();

String typ = md.getColumnTypeName(); }

} catch (SQLException e) {}


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Mapping Database Typesto Java Types

ResultSetmaps

database types

to Java types

ResultSet rset = stmt.executeQuery

("select EMPNO, ENAME, HIREDATE from EMP");

BigDecimal empno = rset.getBigDecimal(1, 2);

String ename = rset.getString(2);Date hiredate = rset.getDate(3);

Col Name

EMPNO

ENAME

HIREDATE

Type

NUMBER

VARCHAR2

DATE

P d S


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Prepared Statements

If you need to execute a statement several times,

with different bind variables:

Use a PreparedStatementobject

Identify bind variables with a ?sign

try {

Connection conn = DriverManager.getConnection();

PreparedStatement pstmt =conn.prepareStatement("update EMP set SAL = ?");


Bi di V i bl d E ti


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Binding Variables and Executing aPreparedStatement

Specify bind variables using themethods setXXX()inPreparedStatement

try {

PreparedStatement pstmt =

conn.prepareStatement("update EMP set SAL = ?");

pstmt.setBigDecimal(1, new BigDecimal(55000));

pstmt.executeUpdate();

pstmt.setBigDecimal(1, new BigDecimal(65000));

pstmt.executeUpdate();


T ti


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Transactions

Transactions are governed by the autoCommit

property in Connection

trueinitially, causing a separate transaction per SQL

statement

If you want to take control:

Connection conn = DriverManager.getConnection();

conn.setAutoCommit(false); // No autocommits now

// Issue SQL statements

conn.commit(); or // Commit transaction

conn.rollback(); // Rollback transaction


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C lli St d P d


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Calling Stored Procedures

getSal(v_ename in varchar2, v_job out varchar2)

return numeric

CallableStatement cs =

conn.prepareCall( "{? = call getSal(?,?)}" );

cs.registerOutParameter(1, Types.NUMERIC);

cs.setString(2, "smith");

cs.registerOutParameter(3, Types.VARCHAR);

cs.executeUpdate();

System.out.println("Smith makes " + cs.getFloat(1) +

" as a " + cs.getString(3));


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Summary

JDBC provides classes and interfaces for

database connectivity

Connect to a database

Perform DML and DDL operations

Prepared statements and stored procedures

16. jdbc&sql pack

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