Java SQL 异常无效游标状态 - 没有当前行答案

【问题标题】：Java SQL Exception Invalid Cursor State - no current rowJava SQL 异常无效游标状态 - 没有当前行
【发布时间】：2013-05-15 22:55:46
【问题描述】：

我一直很难弄清楚这一点。首先，我有一个insertProduct(Product p) 方法，用于检查数据库中是否存在具有指定代码的产品。如果是这样，此方法将显示错误消息。否则，它应该将产品添加到数据库并将其打印到控制台。我不确定我这样做是否正确。

其次，deleteProduct(Product p)方法应该是删除insertProduct方法添加的产品。现在的问题是，当我尝试添加产品时，我不断收到 SQL 异常，然后 deleteProduct 方法只是在每次程序运行时不断删除数据库中的产品，直到没有留下。我不确定这两种方法有什么问题。

控制台输出：

Derby has been started.

Product list:
bvbn    Murach's Beginning Visual Basic .NET        $49.50
cshp    Murach's C#                                 $49.50
java    Murach's Beginning Java                     $49.50
jsps    Murach's Java Servlets and JSP              $49.50
mcb2    Murach's Mainframe COBOL                    $59.50
sqls    Murach's SQL for SQL Server                 $49.50
zjcl    Murach's OS/390 and z/OS JCL                $62.50

First product:
bvbn    Murach's Beginning Visual Basic .NET        $49.50

Last product:
zjcl    Murach's OS/390 and z/OS JCL                $62.50

Product by code: cshp
cshp    Murach's C#                                 $49.50

Insert test: 
java.sql.SQLException: Invalid cursor state - no current row.
Product list:
bvbn    Murach's Beginning Visual Basic .NET        $49.50
cshp    Murach's C#                                 $49.50
java    Murach's Beginning Java                     $49.50
jsps    Murach's Java Servlets and JSP              $49.50
mcb2    Murach's Mainframe COBOL                    $59.50
sqls    Murach's SQL for SQL Server                 $49.50
zjcl    Murach's OS/390 and z/OS JCL                $62.50

Delete test: 
zjcl    Murach's OS/390 and z/OS JCL                $62.50

Product list:
bvbn    Murach's Beginning Visual Basic .NET        $49.50
cshp    Murach's C#                                 $49.50
java    Murach's Beginning Java                     $49.50
jsps    Murach's Java Servlets and JSP              $49.50
mcb2    Murach's Mainframe COBOL                    $59.50
sqls    Murach's SQL for SQL Server                 $49.50

Derby has been shut down.

代码：

   import java.sql.*;

public class DBTesterApp
{
private static Connection connection = null;
private static Product p = null ;
public static void main(String args[])
{
    // get the connection and start the Derby engine
    connection = MurachDB.getConnection();
    if (connection != null)
        System.out.println("Derby has been started.\n");

    // select data from database
    printProducts();
    printFirstProduct();
    printLastProduct();
    printProductByCode("cshp");

    // modify data in the database
     p = new Product("test", "Test Product", 49.50);        
    insertProduct(p);
    printProducts();

    deleteProduct(p);
    printProducts();

    // disconnect from the database
    if (MurachDB.disconnect())
        System.out.println("Derby has been shut down.\n");
 }

 public static void printProducts()
 {

   // Product p = null ;
    try (Statement statement = connection.createStatement();
         ResultSet rs = statement.executeQuery("SELECT * FROM Products"))
    {            


        System.out.println("Product list:");
        while(rs.next())
        {
            String code = rs.getString("ProductCode");
            String description = rs.getString("Description");
            double price = rs.getDouble("Price");

         p = new Product(code, description, price);

            printProduct(p);
        }
        System.out.println();
    }
    catch(SQLException e)
    {
        e.printStackTrace();  // for debugging
    }
  }

  public static void printFirstProduct()
  {

     try(Statement statement = connection.createStatement(
             ResultSet.TYPE_SCROLL_SENSITIVE,
             ResultSet.CONCUR_UPDATABLE);

         ResultSet rs = statement.executeQuery("SELECT * FROM Products")){

         System.out.println("First product:");

         rs.first();
         rs.last();

         if(rs.isFirst() == false){
          rs.previous();
         }

         if(rs.isLast() == false){
             rs.next();
         }

         rs.absolute(1);
         String code = rs.getString(1);
         String description = rs.getString(2);
         double price = rs.getDouble(3);

         p = new Product(code , description , price);
         printProduct(p);
         System.out.println();
     }

     catch(SQLException e){
          e.printStackTrace();
         }
  }

 public static void printLastProduct()
 {
     try(Statement statement = connection.createStatement(
             ResultSet.TYPE_SCROLL_SENSITIVE,
             ResultSet.CONCUR_UPDATABLE);

         ResultSet rs = statement.executeQuery("SELECT * FROM Products")){
         System.out.println("Last product:");


         rs.first();
         rs.last();

         if(rs.isFirst() == false){
          rs.previous();
         }

         if(rs.isLast() == false){
             rs.next();
         }

         rs.absolute(7);
         String code = rs.getString(1);
         String description = rs.getString(2);
         double price = rs.getDouble(3);

         p = new Product(code, description, price);
         printProduct(p);
         System.out.println();



     }

     catch(SQLException e){
         e.printStackTrace();
     }
}

  public static void printProductByCode(String productCode)
  {

       String sql  = 
           "SELECT ProductCode, Description, Price " + 
           "FROM  Products "  +    
           "WHERE ProductCode = ?";

   try(PreparedStatement ps = connection.prepareStatement(sql);){

       ps.setString(1, productCode);
       ResultSet rs = ps.executeQuery();

       if(rs.next()){   
         String description = rs.getString("Description");
         double price = rs.getDouble("Price");
         p = new Product(productCode, description, price);
         System.out.println("Product by code: " + productCode);
         printProduct(p);
        }
          else{
          rs.close();
              }

   }


   catch(SQLException e){
    System.err.println(e);

   }


    System.out.println();
  }

  public static void insertProduct(Product p)
  {
    System.out.println("Insert test: ");


    //check if product code exists in database
   try(Statement statement = connection.createStatement();
        ResultSet rs = statement.executeQuery("SELECT * FROM Products")){

   String code =   rs.getString(1);


  if (p.getCode().equals(code) ){
       System.out.println("Error: This product is already in the database!");
       } 


    else{
    String sql = 
            "INSERT INTO Products (productCode, Description, Price) " +
            "VALUES (?, ?, ?)";

    try(PreparedStatement ps = connection.prepareStatement(sql)){


        ps.setString(1, p.getCode());
        ps.setString(2, p.getDescription());
        ps.setDouble(3, p.getPrice());
        ps.executeUpdate();
       }


     catch(SQLException e){
          System.err.println(e);
      } 


    } //end else

      printProduct(p);
    System.out.println();

   }//end try


        catch(SQLException e ){
        System.out.println(e);
         } 
}

 private static void deleteProduct(Product p)
{
    System.out.println("Delete test: ");

    String sql = "DELETE FROM Products " +
                 "WHERE ProductCode = ?";

    try(PreparedStatement ps = connection.prepareStatement(sql)){
        ps.setString(1, p.getCode());
        ps.executeUpdate();

    }

    catch(SQLException e){
       System.err.println(e);
    }
    // add code that deletes the specified product from the database
    // if a product with the specified code doesn't exist, display an error message

    printProduct(p);
    System.out.println();
}

// use this method to print a Product object on a single line
private static void printProduct(Product p)
{
    String productString =
        StringUtils.padWithSpaces(p.getCode(), 8) +
        StringUtils.padWithSpaces(p.getDescription(), 44) +
        p.getFormattedPrice();

    System.out.println(productString);
}
}

【问题讨论】：

标签： java derby sqlexception

【解决方案1】：

您需要先调用ResultSet.next()，然后才能检索列值。

// check if product code exists in database
try(Statement statement = connection.createStatement();
     ResultSet rs = statement.executeQuery("SELECT * FROM Products")){
if (rs.next()) // THIS is MISSING!
    String code =   rs.getString(1);

上面的代码将毫无例外地运行，但在逻辑上仍然会失败，因为您选择了所有产品并且只是检查数据库返回的第一个产品的代码。检查产品是否已经存在的正确方法是

// check if product code exists in database
try(Statement statement = connection.createStatement();
    ResultSet rs = statement.executeQuery(
        "SELECT * FROM Products WHERE ProductCode = '" + p.getCode() + "'")){
if (rs.next()) {
   System.out.println("Error: This product is already in the database!");
   return;
}

【讨论】：

我试过了，但现在我明白了：java.sql.SQLSyntaxErrorException: Column 'TEST' is either not in any table in the FROM list or appears within a join specification and is outside the scope of the join specification or appears in a HAVING clause and is not in the GROUP BY list. If this is a CREATE or ALTER TABLE statement then 'TEST' is not a column in the target table.
您仍然错过了删除 rs.getString(1);。它就在您添加rs.next() 的上方。检查其余部分。
是的，我删除了它，它仍然给出同样的错误。我不明白rs.next 如果不插入我在 main 方法中创建的产品会如何工作。
在SELECT 查询中为产品代码加上引号。检查更新。（我认为它是一个 int。）
仍然没有运气:(。似乎try方法给出了一个异常，因为还没有列'Test'。该方法应该检查是否传递给该方法的p对象已经在数据库中了。有没有办法我只能从传递给方法的 p 对象中获取产品代码，并执行 if 语句来检查该产品代码是否已经在数据库中，如果没有，则插入产品？

【解决方案2】：

在您的 insertProduct 方法中，您需要在从中获取数据之前调用 ResultSet 上的 next() 方法。

【讨论】：