如何修復Java中的數據庫問題（使用DERBY DB）？

Question

try { 
    Class.forName("org.apache.derby.jdbc.ClientDriver"); 
    Connection con = DriverManager.getConnection("jdbc:derby://localhost:1527/gledi", "root", "root"); 

    String sql = "SELECT MAX(NR) from ROOT.GLEDI"; 

    PreparedStatement pst = con.prepareStatement(sql); 
    ResultSet rs = pst.executeQuery(); 

    if (rs.next()) { 

     String nr1= rs.getString("MAX(NR)"); // here is the whole problem !!!!! how can i fix it 
     text.setText(nr1); 
    } 

    } catch (Exception e) { 
    } 

Answer 1

給它一個名字並查找它。

該列是字符串類型還是數字？

空的catch塊是錯誤的。打印或記錄堆棧跟蹤。你永遠不會知道是否拋出異常。

確定你不想要select count(*) from root.gledi？這個查詢看起來不對我。

您不需要像finally塊那樣關閉Connection,Statement或ResultSet。

您應該將此代碼封裝在一個方法中並給它一個Connection，而不是每次都創建它。

這麼少的代碼，太多的錯誤。

以下是我可能會寫它：

import java.sql.Connection; 
import java.sql.PreparedStatement; 
import java.sql.ResultSet; 
import java.sql.SQLException; 
import java.sql.Statement; 

/** 
* JdbcDemo 
* @author Michael 
* @link http://stackoverflow.com/questions/21205161/how-to-fix-a-db-probblem-in-java-with-derby-db/21205183#21205183 
* @since 1/18/14 9:51 AM 
*/ 
public class JdbcDemo { 
    private static final String SELECT_MAX_ROW_NUMBER = "SELECT MAX(NR) as maxnr from ROOT.GLEDI"; 

    private Connection connection; 

    public JdbcDemo(Connection connection) { 
     this.connection = connection; 
    } 

    public String getMaxRowNumber() { 
     String maxRowNumber = ""; 
     PreparedStatement ps = null; 
     ResultSet rs = null; 
     try { 
      ps = connection.prepareStatement(SELECT_MAX_ROW_NUMBER); 
      rs = ps.executeQuery(); 
      while (rs.next()) { 
       maxRowNumber = rs.getString("maxnr"); 
      } 
     } catch (Exception e) { 
      e.printStackTrace(); // better to log this. 
      maxRowNumber = ""; 
     } finally { 
      close(rs); 
      close(ps); 
     } 

     return maxRowNumber; 
    } 

    // belongs in a database utility class 
    public static void close(Statement st) { 
     try { 
      if (st != null) { 
       st.close(); 
      } 
     } catch (SQLException e) { 
      e.printStackTrace(); // better to log this 
     } 
    } 

    // belongs in a database utility class 
    public static void close(ResultSet rs) { 
     try { 
      if (rs != null) { 
       rs.close(); 
      } 
     } catch (SQLException e) { 
      e.printStackTrace(); // better to log this 
     } 
    } 
}