com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException:未知列'；poi'；在'；其中第'条；

我试图通过从文本字段中读取一个值来创建一个新数据库。在创建数据库之前，我决定检查它是否存在。我的代码如下

 public void createAccount() throws ClassNotFoundException, SQLException {
    try {

        dbName = t12.getText();
        Class.forName("com.mysql.jdbc.Driver");

        connect = DriverManager
                .getConnection("jdbc:mysql://localhost:3306/project?"
                        + "user=root&password=virus");
        statement = connect.createStatement();

        preparedStatement = connect
                .prepareStatement("SELECT COUNT(*) FROM information_schema.tables \n" +
                       "WHERE table_schema = "+dbName+"");
        rs=preparedStatement.executeQuery();
        rs.next();
        int chk = rs.getInt(1);

        if(chk!=1)
        {            
        int resultset = statement.executeUpdate("create database " + dbName );

        connect = DriverManager
                .getConnection("jdbc:mysql://localhost:3306/"+ dbName +"?"
                        + "user=root&password=virus");
        statement = connect.createStatement();

        preparedStatement = connect
                .prepareStatement("create table " + dbName + " (fullname varchar(30),"
                        + "username varchar(20) primary key, password varchar(20),"
                        + "department varchar(30), semester int(2));");
        preparedStatement.executeUpdate();

        preparedStatement = connect
                .prepareStatement("insert into " + dbName + " values(?,?,?,?,?);");

        preparedStatement.setString(1, t11.getText());
        preparedStatement.setString(2, t12.getText());
        preparedStatement.setString(3, p11.getText());
        preparedStatement.setString(4, t13.getText());
        preparedStatement.setString(5, (String)comboBox.getValue());

        preparedStatement.executeUpdate();
        }

        else
        {  
            actiontarget = new Text();
            actiontarget.setFill(Color.FIREBRICK);
            actiontarget.setText("Try another User Name...!");
            actiontarget.setFont(Font.font(null, 15));
            setEffect(new BoxBlur(5, 10, 10));
            Stage usrpagestage = new Stage();
            usrpagestage.setMaxHeight(60);
            usrpagestage.setMaxWidth(200);
            usrpagestage.initStyle(StageStyle.UTILITY);
            usrpagestage.setScene(new Scene(new Warning()));
            usrpagestage.show();

            usrpagestage.setOnCloseRequest(new EventHandler<WindowEvent>() {
                @Override
                public void handle(WindowEvent t) {
                    setEffect(new BoxBlur(0, 0, 0));

                }
            });
        }

    } catch (ClassNotFoundException | SQLException e) {
        throw e;
    } finally {
        close2();
    }

}

为什么会生成此异常？我怎样才能解决它

preparedStatement = connect
                .prepareStatement("SELECT COUNT(*) FROM information_schema.tables \n" +
                       "WHERE table_schema = "+dbName+"");

到

您需要用单引号将where值括起来作为字符串，否则它将抛出错误

这里有一个例子

mysql> SELECT COUNT(*) FROM information_schema.tables where table_schema = users ;
ERROR 1054 (42S22): Unknown column 'users' in 'where clause'

mysql> SELECT COUNT(*) FROM information_schema.tables where table_schema = 'users' ;
+----------+
| COUNT(*) |
+----------+
|        0 |
+----------+

---

您未正确使用

PreparedStatement

使用占位符

？

将值输入查询。通过使用

set…

方法，您可以为查询中的各种参数设置值

这在处理转义字符和sql注入时是安全的

table_schema=poi

的情况不会发生。但是作为

表_name='poi'

在内部进行处理

更改：

preparedStatement = 
  connect.prepareStatement("SELECT COUNT(*) FROM information_schema.tables \n" +
                           "WHERE table_schema = "+dbName+"");
rs=preparedStatement.executeQuery();

String sqlQuery = "SELECT COUNT(*) FROM information_schema.tables \n" +
                  "WHERE table_schema = ?";
preparedStatement = 
  connect.prepareStatement( sqlQuery );

preparedStatement.setString( 1, dbName );

rs=preparedStatement.executeQuery();

至：

preparedStatement = 
  connect.prepareStatement("SELECT COUNT(*) FROM information_schema.tables \n" +
                           "WHERE table_schema = "+dbName+"");
rs=preparedStatement.executeQuery();

String sqlQuery = "SELECT COUNT(*) FROM information_schema.tables \n" +
                  "WHERE table_schema = ?";
preparedStatement = 
  connect.prepareStatement( sqlQuery );

preparedStatement.setString( 1, dbName );

rs=preparedStatement.executeQuery();

你的问题是错误的

preparedStatement = connect
                .prepareStatement("SELECT COUNT(*) FROM information_schema.tables \n" +
                       "WHERE table_schema = "+dbName+"");

那是错误的。始终以单引号传递参数名称，如下所示：

preparedStatement = connect
                .prepareStatement("SELECT COUNT(*) FROM information_schema.tables \n" +
                       "WHERE table_schema = '"+dbName+"'");  

---

这基本上只是重复。