使用java从mysql数据库中读取数据

Firstly, I'm reading the product name and number of products from user using jTextFields. For that product I read the product id and price from database using sql query. But in the below code I display the product price in a jtextField but while running tha file I get query executed successfully but I'm not getting anything in the jtextField.

首先，我正在使用jTextFields从用户那里读取产品名称和产品数量。对于该产品，我使用sql查询从数据库读取产品id和价格。但是在下面的代码中，我在jtextField中显示产品价格，但是在运行这个文件时，我成功地执行了查询，但是在jtextField中我什么也没有得到。

And please check the sql query and resultset use, table name is "item" and database name is "myshop", I declared variables globelly and this code is in a jButton's 'ActionPeformed" part.

请检查sql查询和resultset的使用，表名是“item”，数据库名是“myshop”，我全局声明变量，这段代码在jButton的‘ActionPeformed’部分。

String item_name=name.getText();
int item_no=Integer.parseInt(no.getText());
String sql="SELECT id,price FROM item WHERE item.name='item_name'";
try{       
Class.forName("com.mysql.jdbc.Driver");
Connection con(Connection)DriverManager.getConnection("jdbc:mysql://localhost:3306/myshop","root","mysql");
java.sql.Statement stmt=con.createStatement();
if (stmt.execute(sql)) {
rs = stmt.getResultSet();
JOptionPane.showMessageDialog(this, "succes","executed query",JOptionPane.PLAIN_MESSAGE);
} else {
System.err.println("select failed");}
int idIndex   = rs.findColumn("id");
int priceIndex    = rs.findColumn("price");

while(rs.next()){
item_id=rs.getInt(idIndex);
item_price=rs.getInt(priceIndex);
jTextField1.setText(""+item_price);//displaying product price in a jTextField1
jTextField2.setText(""+item_id);//displaying product id in a jTextField2  
  }
}
catch(Exception e){
JOptionPane.showMessageDialog(this, e.getMessage());
}

5 个解决方案

#1

you would need to take item_name as param and put in quotes,

你需要将item_name作为参数并加上引号，

String sql="SELECT id,price FROM item WHERE item.name='"+ item_name+"'";

#2

This line should be

这条线应该是

String sql="SELECT id,price FROM item WHERE item.name='item_name'";

like this

像这样

String sql="SELECT id,price FROM item WHERE item.name='"+item_name+"'";

#3

Use a PreparedStatement so you don't have to worry about delimiting all the variables:

使用一个PreparedStatement，这样你就不用担心所有变量的限制:

String sql="SELECT id, price FROM item WHERE item.name = ?";
PreparedStatement stmt = connection.prepareStatement(sql);
stmt.setString( 1, item_name);
ResultSet rs = stmt.executeQuery();

Then the prepared statement will replace the variable for you with the proper quotes.

然后，准备好的语句将用正确的引号替换变量。

#4

Try to avoid this type of mistake by using PreparedStatement

通过使用PreparedStatement来避免这种错误

String sql="SELECT id,price FROM item WHERE item.name=?";  
PreapredStatement ps = con.prepareStatement(sql);
ps.setString(1,item_name); 
ResultSet rs = ps.executeQuery();

Use of PreparedStatement also prevent SQL injection attack.

使用PreparedStatement也可以防止SQL注入攻击。

#5

try this code .

试试这个代码。

Connection con = DriverManager.getConnection("jdbc:mysql://localhost:3306/myshop","root","mysql");
            PreparedStatement pt=con.prepareStatement("SELECT id,price FROM item WHERE item.name=?");
            pt.setString(1,"item_name");
          ResultSet rs;
           if(pt.execute())
           {
                rs=pt.getResultSet();
               JOptionPane.showMessageDialog(this, "succes","executed query",JOptionPane.PLAIN_MESSAGE);
          }
           else {
    System.err.println("select failed");
           }

        while(rs.next()){
    item_id=rs.getInt(1);
    item_price=rs.getInt(2);
    jTextField1.setText(""+item_price);//displaying product price in a jTextField1
    jTextField2.setText(""+item_id);//displaying product id in a jTextField2    

        }

#1