Java 当一列为空时，无法在数据库中找到项

我有一个程序，它要求我输入产品的ID，然后搜索数据库以显示特定产品的详细信息

对于数据库中除ProductID2和ProductID4之外的每个项目，代码都可以正常工作，因为它们没有存储在其中一列（PRODUCT_类型）中的数据

我的代码中有一个if语句，它说：

但是我不明白为什么当产品类型为null时它找不到产品，但是当描述为null时它仍然可以找到产品

    try{        
        String searchCode = "SELECT PRODUCTS.PRODUCT, PRODUCTS.COST, PRODUCTS.DESCRIPTION, PRODUCT_STOCK.STOCKITEM , PRODUCT_STOCK.STOCKLEVEL FROM PRODUCT_TYPE INNER JOIN (PRODUCTS FULL OUTER JOIN PRODUCT_STOCK ON PRODUCTS.PRODUCTID = PRODUCT_STOCK.STOCKITEM) ON PRODUCT_TYPE.PRODTYPEID = PRODUCTS.PRODUCT_TYPE WHERE PRODUCTID = " + inputValue + ";";
        // sql statement searches the database for the required data

        Statement statement = dbConnection.createStatement();
        ResultSet result = statement.executeQuery(searchCode);
        boolean found = false;
        found = result.next();

        if (!found) { //if no data is found then display the error message and restart the product search method
            System.out.println("That product couldn't be found, please try again.");
            searchProduct();
        }
        else if (result.getString("PRODUCT_TYPE") == null && result.getString("DESCRIPTION ") == null)
        {
            System.out.println("Item ID: " + result.getString("STOCKITEM") + ". \nProduct name: " + result.getString("PRODUCT") + ". \nCost: £" + result.getString("COST") + ". \nStock level: " + result.getString("STOCKLEVEL") + ". \nProduct type: null.");
        }
        else if (result.getString("PRODUCT_TYPE") == null && result.getString("DESCRIPTION") != null){
            System.out.println("Item ID: " + result.getString("STOCKITEM") + ". \nProduct name: " + result.getString("PRODUCT") + ". \nCost: £" + result.getString("COST") + ". \nStock level: " + result.getString("STOCKLEVEL") + ". \nDescription: " + result.getString("DESCRIPTION") + ". \nProduct type: null.");
            }
        else if (result.getString("PRODUCT_TYPE") != null && result.getString("DESCRIPTION") == null){
                System.out.println("Item ID: " + result.getString("STOCKITEM") + ". \nProduct name: " + result.getString("PRODUCT") + ". \nCost: £" + result.getString("COST") + ". \nStock level: " + result.getString("STOCKLEVEL") +  ". \nDescription: null. \nProduct type: " + result.getString("PRODTYPE_DESC") + ".");
            }
        else if (result.getString("PRODUCT_TYPE") != null && result.getString("DESCRIPTION") != null){
            System.out.println("Item ID: " + result.getString("STOCKITEM") + ". \nProduct name: " + result.getString("PRODUCT") + ". \nCost: £" + result.getString("COST") + ". \nStock level: " + result.getString("STOCKLEVEL") + ". \nDescription: " + result.getString("DESCRIPTION") + ". \nProduct type: " + result.getString("PRODTYPE_DESC") + ".");
            }
        }
    catch (Exception e){ // if the product can't be found then display the error message and restart the product search method
        System.out.println("Could not find product.");
        System.out.println(e.toString());
        searchProduct();
    }

    //goes back to menu once the method has finished
    askUser();
}

任何帮助都将不胜感激。谢谢。

你有打字错误吗

result.getString（“DESCRIPTION”）==null

应该是

result.getString（“DESCRIPTION”）==null

---

请注意当前版本中

“Description”

中的额外空格。

我已经更改了，它仍然在说“找不到该产品。我认为这是SQL查询中的问题，但我很难理解，因此我不知道是什么原因导致程序能够找到具有PRODUCT_TYPE值但无法找到null值的产品。您的SQL正在对products.PRODUCT_TYPE进行连接，该类型为null。这意味着连接不会连接任何东西，因为没有任何东西可以与null匹配。那么有什么方法可以解决这个问题吗？没有任何东西可以链接到另一个表，该表是该表中关于其他产品类型表的唯一列。

    try{        
        String searchCode = "SELECT PRODUCTS.PRODUCT, PRODUCTS.COST, PRODUCTS.DESCRIPTION, PRODUCT_STOCK.STOCKITEM , PRODUCT_STOCK.STOCKLEVEL FROM PRODUCT_TYPE INNER JOIN (PRODUCTS FULL OUTER JOIN PRODUCT_STOCK ON PRODUCTS.PRODUCTID = PRODUCT_STOCK.STOCKITEM) ON PRODUCT_TYPE.PRODTYPEID = PRODUCTS.PRODUCT_TYPE WHERE PRODUCTID = " + inputValue + ";";
        // sql statement searches the database for the required data

        Statement statement = dbConnection.createStatement();
        ResultSet result = statement.executeQuery(searchCode);
        boolean found = false;
        found = result.next();

        if (!found) { //if no data is found then display the error message and restart the product search method
            System.out.println("That product couldn't be found, please try again.");
            searchProduct();
        }
        else if (result.getString("PRODUCT_TYPE") == null && result.getString("DESCRIPTION ") == null)
        {
            System.out.println("Item ID: " + result.getString("STOCKITEM") + ". \nProduct name: " + result.getString("PRODUCT") + ". \nCost: £" + result.getString("COST") + ". \nStock level: " + result.getString("STOCKLEVEL") + ". \nProduct type: null.");
        }
        else if (result.getString("PRODUCT_TYPE") == null && result.getString("DESCRIPTION") != null){
            System.out.println("Item ID: " + result.getString("STOCKITEM") + ". \nProduct name: " + result.getString("PRODUCT") + ". \nCost: £" + result.getString("COST") + ". \nStock level: " + result.getString("STOCKLEVEL") + ". \nDescription: " + result.getString("DESCRIPTION") + ". \nProduct type: null.");
            }
        else if (result.getString("PRODUCT_TYPE") != null && result.getString("DESCRIPTION") == null){
                System.out.println("Item ID: " + result.getString("STOCKITEM") + ". \nProduct name: " + result.getString("PRODUCT") + ". \nCost: £" + result.getString("COST") + ". \nStock level: " + result.getString("STOCKLEVEL") +  ". \nDescription: null. \nProduct type: " + result.getString("PRODTYPE_DESC") + ".");
            }
        else if (result.getString("PRODUCT_TYPE") != null && result.getString("DESCRIPTION") != null){
            System.out.println("Item ID: " + result.getString("STOCKITEM") + ". \nProduct name: " + result.getString("PRODUCT") + ". \nCost: £" + result.getString("COST") + ". \nStock level: " + result.getString("STOCKLEVEL") + ". \nDescription: " + result.getString("DESCRIPTION") + ". \nProduct type: " + result.getString("PRODTYPE_DESC") + ".");
            }
        }
    catch (Exception e){ // if the product can't be found then display the error message and restart the product search method
        System.out.println("Could not find product.");
        System.out.println(e.toString());
        searchProduct();
    }

    //goes back to menu once the method has finished
    askUser();
}