Question

我正在尝试更新名为product的数据库中的数量列。我想要从30到29的数量更新饮料芬达。一旦我开始工作，我可以对所有其他产品做同样的事情。任何帮助将不胜感激。谢谢这是我的DB

＆＃39; 7＆＃39;，＆＃39;芬达＆＃39;，＆＃39;芬达橙＆＃39;，＆＃39; 2＆＃39;，＆＃39;冷饮＆＃39;，＆＃39;可口可乐＆＃39; 30＆＃39; ＆lt; ==数量列

到目前为止，这是我的代码。

// Update Stock
    public void update() 
    {
        Connection conn = null;

        try
        {
        Class.forName("com.mysql.jdbc.Driver");
        conn = (Connection) DriverManager.getConnection(DB_URL, USER_NAME, PASSWORD);
        conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/team_project_schema?useSSL=false", "root" , "password");

            String query = "UPDATE product SET Quantity = Quantity - 1 WHERE ProductName = 'Fanta' and Quantity > 0";
            PreparedStatement pstmt = conn.prepareStatement(query);
            pstmt.setString(7, "Fanta");
            pstmt.executeUpdate();

        } catch (Exception e) 
        {
            // If we don't have a error, close the connection!
            System.err.println("Exception: " + e.getMessage());

        } finally 
        {
            try 
            {
                if (conn != null) 
                {
                    conn.close();
                }
            } catch (SQLException e) 
            {

            }
        }

    }

package testers;

import classes.Stock;
import java.util.*;

public class StockTester 
{

    public static void main(String[] args)
    {
        // create object of Stock
        Stock stock = new Stock();


        // print out stock from DB
        System.out.println("Reading All Products From DB... ");
        System.out.println();
        ArrayList <Stock> allStock = stock.getProducts();
        Iterator<Stock> Iterator = allStock.iterator();

        while (Iterator.hasNext())
        {
            Stock displayStock = Iterator.next();
            System.out.println(displayStock.getProductID() + " \t" + displayStock.getProductName()
            + " \t" + displayStock.getProductDesc() + " \t" + displayStock.getSection()
            + " \t" + displayStock.getSupplierName());
        }
    }
}

Answer 1

我这样做对我来说最简单：

date

你只需要写下新数量而不是你想要多少。例如：29不是-1;

Answer 2

您可以将productId发送到update方法并将其作为参数发送到Prepared Satement。

public void update() 
{
   ...
   String query = "UPDATE product SET Quantity = Quantity - 1 WHERE ProductID = ? and Quantity > 0";
   PreparedStatement pstmt = conn.prepareStatement(query);
   pstmt.setString(1, this.productID);
   pstmt.executeUpdate();
   ...
}

在您的测试类中，您可以像这样更新。

while (Iterator.hasNext())
{
  Stock displayStock = Iterator.next();
  displayStock.update();
}

Answer 3

我在SQL中没有看到任何参数，因此下面的代码会失败：

pstmt.setString(7, "Fanta");

你的SQL应该看起来像：

String query = "UPDATE product SET Quantity = Quantity - ? WHERE ProductName = ?";
pstmt.setInt(1, 1); // the first parameter is the index of the variable in your SQL
pstmt.setString(2, "Fanta");

更新：让我们假设您有一个“订单”项，它包含“LineItems”的“列表”

public void updateStockCounts(Order order) {
    Connection conn = getConnection(); // get a connection to the database
    conn.setAutoCommit(false); // dont update the db automatically, let me do it
    String sql = "UPDATE product SET Quantity = Quantity - ? WHERE ProductName = ?";
    PreparedStatement pStmt = conn.prepareStatement(sql);

    for(LineItem lineItem : order.getLineItems()) {
        pStmt.setInt(1, lineItem.getQuantity()); // this is the quantity sold
        pStmt.setString(2, lineItem.getStockName()); // this the name of the product 'Fanta' for example
        pStmt.executeUpdate();
    } // go on to the next lineItem if there are any more
    conn.commit(); // update all processed items now.
}

当然还有其他事情要做，例如错误检查，例如，与数据库的连接可能会中断。

如何更新数据库

3 个答案: