[英]Multiple sql queries in one connection using java
我正在尝试在单个数据库连接中进行不同的查询，但找不到方法。 该查询是根据之前的结果进行查询的。 这是代码：
String query_disco="SELECT * FROM disco";
String name = "";
String adress="";
String web="";
int disco_id=0;
String query_disco_var="SELECT * FROM disco_var";
float price=0;
String style="";
String hin="";
String hout="";
String party_name="";
int day=0;
String docType="";
String docTable="";
String docBody="";
Calendar calendar = Calendar.getInstance();
try {
DAO a= new DAO();
ResultSet b =a.executarSQL(query_disco);
day = calendar.get(Calendar.DAY_OF_WEEK);
docType ="<!DOCTYPE HTML PUBLIC\"--�//W3C//DTD HTML 4.0 " +"Transitional//EN\">\n";
docBody =
"<HTML>\n"+
"<HEAD><TITLE>HelloWorld!</TITLE></HEAD>\n"+
"<BODY BGCOLOR=\"#FDF5E6\">\n"+
"<H1>Disco</H1>\n"+
"<table border=1>"+
"<tr> <th> name </th> <th> adress </th> <th> web </th> <th> price </th>" +
"<th> style </th> <th> horari </th> <th> party name </th> </tr>";
while(b.next()){
disco_id=b.getInt("disco_id");
name=b.getString("name");
adress=b.getString("adress");
web=b.getString("web");
query_disco_var = query_disco_var + " WHERE disco_id = " + disco_id + " AND day = " + day;
DAO d= new DAO();
ResultSet c = d.executarSQL(query_disco_var);
price=c.getFloat("price");
style=c.getString("style");
hin=c.getString("hin");
hout=c.getString("hout");
party_name=c.getString("party_name");
docTable = docTable + "<tr> <td> "+name+" </td> <td> "+adress+" </td> <th>" +web+ "</td> </tr>"+
price+ "</td> <td>" +style+ "</td> <td>" +hin+ "-" +hout+ "</td> <td>" +party_name+ "</td> </tr>";
query_disco_var="SELECT * FROM disco_var";
}
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
out.println(docType+docBody+docTable+
"</table>" +
"</BODY></HTML>");
}
/**
* @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
*/
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
}
}
与de db的连接与此脚本配合使用：
public class DAO {
private Connection connexio;
private Statement sentencia;
/**
*
* @throws Exception
*/
public DAO() throws Exception {
String user="trmcfunt";
String password="44445555";
Class.forName("com.mysql.jdbc.Driver").newInstance();
connexio=
DriverManager.getConnection("jdbc:mysql://localhost/iguanaparty_db_test?user="+user+"&password="+password);
sentencia=connexio.createStatement();
}
/**
*
* @param query
* @return
* @throws SQLException
*/
public ResultSet executarSQL(String query)throws SQLException{
return sentencia.executeQuery(query);
}
/**
*
* @throws SQLException
*/
public void desconnectarBD() throws SQLException {
sentencia.close();
connexio.close();
}
}
我正在得到这样的exepcions：
java.sql.SQLException: Before start of result set
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1074)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:988)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:974)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:919)
at com.mysql.jdbc.ResultSetImpl.checkRowPos(ResultSetImpl.java:854)
at com.mysql.jdbc.ResultSetImpl.getStringInternal(ResultSetImpl.java:5772)
at com.mysql.jdbc.ResultSetImpl.getString(ResultSetImpl.java:5692)
at com.mysql.jdbc.ResultSetImpl.getFloat(ResultSetImpl.java:2615)
at com.mysql.jdbc.ResultSetImpl.getFloat(ResultSetImpl.java:2635)
at controllers.HelloWorld.doGet(HelloWorld.java:87)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:99)
at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408)
at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1023)
at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:589)
at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:310)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
at java.lang.Thread.run(Thread.java:724)
问候。
ResultSet c = d.executarSQL(query_disco_var);
price = c.getFloat("price");
从结果集中获取值之前，必须调用next（）将光标放在第一行上。 否则，如异常中所述，您将在结果集开始之前：
ResultSet c = d.executarSQL(query_disco_var);
if (c.next()) {
price = c.getFloat("price");
另外，了解准备好的语句 ，因为通过将参数串联在一起将参数传递给查询会使您的代码容易受到SQL注入攻击，因此，例如，其中一个字符串参数包含单引号，广告就会立即中断。
您的ResultSet c永远不会移到第一个结果（如果有）。 您应该使用类似以下的内容包装从c读取的代码
if (c.next()) {
// your original code here
} else {
// whatever you want to do in this case
}
另外，您可能不得不考虑如果发现多个结果该怎么办。
执行任何SQL后，游标将位于第一行之前 。 这就是为什么要执行该命令，因为没有可用数据。
试试c.next(); 在执行第二个SQL查询之后。 这会将光标定位在第一个结果上。 请记住，如果没有返回数据，则此调用将返回false并且随后的列提取都将失败，因此您应该抓住这一点。
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