我与Db2 9新特性的零距离体验(4)

作者: 宗一 出处:　 

其他查询操作
SQL 程序员常常编写根据某种条件限制从 DBMS 返回的行的查询。例如，SQL 查询限制从 "CUSTOMER" 表中检索的行，使之只包括那些具有某种状态的客户。在这个例子中，客户的状态可在 SQL VARCHAR 列中捕捉。但是，如果您想根据某种应用于 XML 列中数据的条件对搜索进行限制，那么应该怎么做呢？SQL/XML 的 XMLExists 函数为完成该任务提供了一种手段。 

通过 XMLExists 可以在 XML 文档中找到一个元素，并测试它是否满足某个特定的条件。如果用在 WHERE 子句中，则 XMLExists 可以限制返回的结果，使之只包括那些包含具有特定 XML 元素值的 XML 文档的行（换句话说，指定的值等于 "true"）。 

例如您想找到居住在具有特定邮政编码的地区的所有客户的姓名。您也许还记得，" CUSTOMER " 表的一个 XML 列中存储了客户的地址（包括邮政编码）。通过使用 XMLExists，可以从 XML 列中搜索目标邮政编码，并相应地限制返回的结果集。下面的 SQL/XML 查询返回居住在邮政编码为 95116 的地区的客户的姓名： 

根据 XML 元素值限制结果

select name from CUSTOMER
where xmlexists('$c/Client/Address[zip="95116"]'
passing CUSTOMER.INFO as "c")

如果我们想将 XML 值投影到返回的结果集。换句话说，我们要从 XML 文档中检索一个或多个元素值。有很多方法可以做这件事。首先我们使用 XMLQuery 函数来检索一个元素的值，然后使用 XMLTable 函数来检索多个元素的值，然后将这些映射到一个 SQL 结果集的列。 

比如如何创建一个列出 具有某种状态的客户的 email 地址的报告。我们可以通过XMLQuery 函数来完成这项任务：
检索符合条件的客户的 email 信息

select xmlquery('$c/Client/email'
passing info as "c")
from CUSTOMER
where status = 'good'

更新操作

DB2 允许用 SQL UPDATE 语句或通过使用系统提供的存储过程（DB2XMLFUNCTIONS.XMLUPDATE）来更新 XML 列。不管使用哪种方式，对 XML 列的更新都发生在元素级。然而，使用存储过程更新 XML 数据的程序员不需要提供整个 XML 文档给 DB2；他们只需指定要更新的 XML 元素。发出 UPDATE 语句的程序员则需要指定整个文档（而不仅仅是要更改的元素）。

例如，如果要发出一条 UPDATE 语句来更改某个特定客户的联系方式信息中的 email 地址，就必须在 XML 列中提供全部联系方式信息，而不仅仅是新的 email 元素值。

update clients set contactinfo=(
xmlparse(document '[e-mail]' ) )
where id = 3227

本例中更新xml类型字段的类为updatexml.java，该类通过提供cid来更新CUSTOMER表中的客户信息。

import java.sql.*;
import java.io.*;
import java.util.*;
import org.xml.sax.*;
import javax.xml.parsers.*;
import org.apache.xerces.dom.DOMImplementationImpl;
import org.w3c.dom.*;
import org.apache.xml.serialize.*;
public class updatexml
...{
/**//** private members **/
private static Connection conn;
private static PreparedStatement sStmt;
private static PreparedStatement uStmt;
private static ResultSet rs;
/**//*needed for properties file*/
static Properties db2ConnProps = new Properties();
static Properties fileinputProps = new Properties();
/**//* Default Constructor */
public updatexml() ...{}
/**//* Main Driver*/
public static void main(String[] args)
...{
String db,pID, userName, passwd, host, port, pInfo, input, newDoc, xpath, fileName;
String inputfile = " ";
boolean inputfilevalidate=false;
String rollbackCommit="rollback";
String interactive = "";
boolean exit, update;
boolean validate=false;
exit=update=false;
int option;
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String [] productInfo = ...{"name", "details", "price", "weight" };
String [] productValues = new String[4];
newDoc=pInfo=host=port=db=pID=userName=passwd=input=null;
/**//** print user options **/
try
...{
/**//* load the contents of properties file in case of missing arguments*/
db2ConnProps.load(new FileInputStream 
("F:\\eclipse\\WorkPlace\\DB2XML\\src\\db2Conn.properties"));
db=db2ConnProps.getProperty("databaseName");
userName=db2ConnProps.getProperty("userName");
passwd=db2ConnProps.getProperty("password");
host=db2ConnProps.getProperty("hostName");
port=db2ConnProps.getProperty("portNumber");
rollbackCommit=db2ConnProps.getProperty("rollbackCommit");
interactive=db2ConnProps.getProperty("interactive");
/**//** establish DB connection **/
conn=db2Conn.get(db,userName,passwd,host,port);
try
...{conn.setAutoCommit(false);}
catch (java.sql.SQLException e)
...{
e.printStackTrace();
System.exit(-1);
}
/**//* prints current connection status*/
System.out.println();
System.out.println(" This sample updates xml information from a table.");
System.out.println();
System.out.println(" Connect to '"+db+"' database using JDBC Universal type 4 driver");
System.out.println(" Connection: com.ibm.db2.jcc");
try
...{ db2ConnProps.load(new FileInputStream 
("F:\\eclipse\\WorkPlace\\DB2XML\\src\\db2Conn.properties"));}
catch(IOException io) ...{System.out.println(io.getMessage());}
catch(NullPointerException ne) ...{System.out.println("NullPointerException");}

/**//** get product id if not interactive, else request user input**/
if (interactive.equals("no"))
...{
pID = db2ConnProps.getProperty("pID");
System.out.println();
System.out.println(" Using default product ID of "+pID);
}
else
...{
pID=db2ConnProps.getProperty("pID");
System.out.println();
System.out.print(" Enter Product ID #[default is '1000']:");
pID = br.readLine();
}
/**//*If user just presses enter, then use default value*/
if(pID.length()==0)
...{
System.out.println(" Nothing entered, thus, using default value.");
pID=db2ConnProps.getProperty("pID");
}
else;
/**//** prompt for valid product ID **/
while((pInfo=getProduct(pID))==null)
...{
System.out.println();
System.out.println(" No Such Product ID can be found. Do you want to use default");
System.out.print(" value of "+db2ConnProps.getProperty("pID")+" [y/n] or q to quit: ");
input=br.readLine();
/**//** if update, parse and update new product **/
if(input.equals("y") || input.equals("Y"))
...{
pID=db2ConnProps.getProperty("pID");
System.out.println();
System.out.println(" Now using pid="+pID);
}
else if(input.equals("q") || input.equals("Q"))
...{
System.out.println(" Quitting program! ");
System.exit(-1);
}
else
...{
System.out.println();
System.out.print(" Please re-enter Product #: ");
pID = br.readLine();
}
}//end while 
/**//** insert product info into a DOM and use DOM API to extract element values **/
domUtility.initializeFromString(pInfo);
/**//** Print the product description currently **/
System.out.println();
System.out.println(" Current contents of DESCRIPTION in the PRODUCTS table");
System.out.println(" for pid='"+pID+"'");
System.out.println();
System.out.println(domUtility.toString("/product"));
/**//** print product update options **/
printProduct(productInfo);
if (interactive.equals("no"))
...{ System.out.println(" Just changing #[4] weight");
input = "4";
}
else
...{ input = ""; }
while(!(input.equals("0") | input.equals("1") | input.equals("2") | 
input.equals("3") | input.equals("4")))
...{
System.out.println();
System.out.print(" Enter # of the Item to change (or zero to quit): ");
input =br.readLine();
}
option= Integer.parseInt(input);
/**//** update product value or exit **/
while(!(option == 0))
...{
System.out.println();
switch (option)
...{
case 0: exit=true;
break ;
case 1: System.out.print(" Enter new Value for \"name\": ");
input=br.readLine();
domUtility.setValue("/product/description/name/text()",input);
break;
case 2: System.out.print(" Enter new Value for \"details\": ");
input=br.readLine();
domUtility.setValue("/product/description/details/text()",input);
break;
case 3: System.out.print(" Enter new Value for \"price\": ");
input=br.readLine();
domUtility.setValue("/product/description/price/text()",input);
break;
case 4:
if (interactive.equals("no"))
...{
input="5";
System.out.println();
System.out.println(" Using default Product Weight of 5.");
System.out.println();
option = 0;
}
else
...{ System.out.print(" Enter new Value for \"weight\": ");
input=br.readLine();
}
domUtility.setValue("/product/description/weight/text()",input);
break;
default: System.out.print(" Invalid Entry, please try again");
break;
}//end switch 

/**//** select next item to update **/
if (interactive.equals("no"))
...{ input = "0"; }
else
...{ input = "";
printProduct(productInfo);
}
while(!(input.equals("0") | input.equals("1") | input.equals("2") | 
input.equals("3") | input.equals("4")))
...{
System.out.print(" Enter # of the Item to change (or zero to quit): ");
input =br.readLine();
}
option= Integer.parseInt(input);
}//end while(!(option == 0)) 
System.out.println();
System.out.println(" Updating the record in the Products table using the above data");
System.out.println(" updateProd(domUtility.toString(\"/product\"),pID)");
System.out.println();
updateProd(domUtility.toString("/product"),pID);
/**//** Print the product description currently--before rollback **/
pInfo=getProduct(pID);
domUtility.initializeFromString(pInfo);
System.out.println(" After update, contents of DESCRIPTION in the PRODUCTS table");
System.out.println(" for pid='"+pID+"'");
System.out.println();
System.out.println(domUtility.toString("/product"));

//rollback and then print the contents 
if (rollbackCommit.equals("rollback"))
...{
System.out.println(" Rollback the transaction.");
try
...{ conn.rollback(); }
catch (Exception e)
...{ JdbcException jdbcExc = new JdbcException(e, conn);
jdbcExc.handle(); }
}
else ;
/**//** Print the product description currently--After rollback **/
pInfo=getProduct(pID);
domUtility.initializeFromString(pInfo);
System.out.println();
System.out.println(" After rollback, contents of DESCRIPTION in the PRODUCTS table");
System.out.println(" for pid='"+pID+"'");
System.out.println();
System.out.println(domUtility.toString("/product"));
/**//** close connections **/
try ...{conn.commit();}
catch(SQLException ex)
...{
System.err.println("SQLException information");
while(ex!=null)
...{
System.err.println ("Error msg: " + ex.getMessage());
System.err.println ("SQLSTATE: " + ex.getSQLState());
System.err.println ("Error code: " + ex.getErrorCode());
ex.printStackTrace();
ex = ex.getNextException(); // For drivers that support chained exceptions 
}
}
System.out.println(" Disconnect from the '"+db+"' database");
closeConn();
System.out.println(" Disconnected.");
}
catch(NullPointerException ne)...{System.out.println("NullPointerException main");}
catch(IOException io)...{io.printStackTrace();}
}//main 
public static void setConn(Connection c)
...{
conn=c;
}
/**//* Get product from database */
public static String getProduct(String pID)
...{
String pInfo=null;
try
...{
/**//** Prepare Statement **/
sStmt= conn.prepareStatement("Select INFO from CUSTOMER where cid=?");
sStmt.setString(1,pID);
rs=sStmt.executeQuery();
if (rs.next())
...{
pInfo=rs.getString(1);
}
else;
}
catch(SQLException sqle)
...{
System.out.println("Error Msg: "+ sqle.getMessage());
System.out.println("SQLState: "+sqle.getSQLState());
System.out.println("SQLError: "+sqle.getErrorCode());
System.out.println("Rollback the transaction and quit the program");
System.out.println();
try ...{ conn.rollback(); }
catch (Exception e)
...{
JdbcException jdbcExc = new JdbcException(e, conn);
jdbcExc.handle();
}
System.out.println();
System.exit(1);
}
return pInfo;
}//end getProduct 
/**//* Update Product */
public static void updateProd(String newDoc, String pID)
...{
int recordUpdated=0;
try
...{
/**//** Prepare Statement **/
uStmt= conn.prepareStatement("Update CUSTOMER set INFO = ? where PID=?");
uStmt.setString(1,newDoc);
uStmt.setString(2,pID);
if(uStmt.executeUpdate()!=1)
System.out.println("Product could not be updated");
else ;
}
catch(SQLException sqle)
...{
System.out.println("Error Msg: "+ sqle.getMessage());
System.out.println("SQLState: "+sqle.getSQLState());
System.out.println("SQLError: "+sqle.getErrorCode());
System.out.println("Rollback the transaction and quit the program");
System.out.println();
try ...{ conn.rollback(); }
catch (Exception e)
...{
JdbcException jdbcExc = new JdbcException(e, conn);
jdbcExc.handle();
}
System.exit(1);
}
}//end updateProd 
/**//* Print Product Update Values */
public static void printProduct(String [] pInfo)
...{
String pValue, xpath;
pValue = xpath = null;
System.out.println();
System.out.println(" Product ID="+domUtility.getValue("");
for(int i=0; i < pInfo.length; i++)
...{
xpath="/Client/description/"+pInfo[i]+"/text()";
pValue = domUtility.getValue(xpath);
if(pValue != null)
System.out.println(" ["+(i+1)+"] "+pInfo[i].toString()+" : "+ pValue);
}
}//end printProduct 
/**//* Close conections */
public static void closeConn()
...{
try
...{
conn.close();
sStmt.close();
uStmt.close();
}
catch(SQLException sqle)
...{
System.out.println(sqle.getMessage());
System.out.println(sqle.getSQLState());
System.out.println(sqle.getErrorCode());
}
}//end closeConn 
}

删除 XML 数据 

删除包含 XML 列的行很简单。SQL DELETE 语句允许通过 WHERE 子句识别（或限制）要删除的行。该子句可以包括简单的谓词来标识非 XML 列值或包括 SQL/XML 函数来标识包含在 XML 列中的 XML 元素值。

例如，下面展示了如何删除客户 CID 为 3227 的客户的所有信息：

delete from CUSTOMER 
where cid = 1000 

还记得怎样限制 SQL SELECT 语句，使之仅返回居住在邮政编码为 95116 的地区的客户的行吗？如果还记得的话，很容易知道如何删除与那些客户相关的行。下面看看如何使用 XMLExists 来做这件事： 

删除居住在特定地区的客户的数据

delete from clients 
where xmlexists('$c/Client/Address[zip="95116"]' 
passing CUSTOMER.INFO as "c");