当一列为空时，无法在数据库中找到项目

Question

我有一个程序要求我输入产品的 ID，然后搜索数据库以显示有关该特定产品的详细信息。

该代码适用于数据库中除 productID 2 和 productID 4 之外的每个项目，因为它们没有数据存储在某一列 (PRODUCT_TYPE) 中。

我的代码中有一条 if 语句：

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.");
}

但我不明白为什么如果产品类型为空，它无法找到产品，但如果描述为空，仍然可以找到它。

    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();
}

任何帮助将不胜感激。谢谢。

Answer 1

你有错别字吗？

result.getString("DESCRIPTION ") == null

应该是

result.getString("DESCRIPTION") == null

注意您当前版本中"Description " 中的额外空格。