java如何使用JDBC从mySQL获取特定列？

Question

    PreparedStatement ps2;
    ResultSet rs2;

    String username = jTextField_username.getText();
    String userpassword = jTextField_userpassword.getText();

    String sql = "SELECT distinct kalanizin from users where username =?";
    String[] resultsetarray = null;

    try {

       ps2 = MyConnection.getConnection().prepareStatement(sql);
       ps2.setString(1, username);

       rs2 = ps2.executeQuery();
       String result = rs2.getString(0); // i want this String to hold the first result

大家好。我有一个问题一直在打我的头。正如标题中所说，使用JDBC时，我希望从数据库中获取一列。比如说

Select * from users where username = "blabla"

我希望Java执行这个查询并获得它找到的第一行。然后放入字符串或整数。正如你在上面看到的我糟糕的代码，它没有做到这一点。我试图通过使用语句来实现这一点，但仍然不起作用。有人能帮我吗？我们将不胜感激。 对不起，我的变量名太糟糕了，并且使用我的本地语言作为变量

Answer 1

我的做法是：

1）限制查询。您可以将限制作为1添加到查询中。我将鼓励您阅读，而不是提供样品

2）例如，仅获取所需的列

select u.age from users u where u.username=? 

然后根据数据类型，您可以从结果集中获取它

Answer 2

如果知道列名，可以使用^{}等方法代替getString(columnIndex)：

Retrieves the value of the designated column in the current row of this ResultSet object as a String in the Java programming language.

columnLabel - the label for the column specified with the SQL AS clause. If the SQL AS clause was not specified, then the label is the name of the column

Answer 3

您缺少ResultSet#next调用

根据示例代码，您可以执行以下操作：

PreparedStatement ps2;
ResultSet rs2;

String username = jTextField_username.getText();
String userpassword = jTextField_userpassword.getText();

String sql = "SELECT distinct kalanizin from users where username =?";
String[] resultsetarray = null;

try {
   ps2 = MyConnection.getConnection().prepareStatement(sql);
   ps2.setString(1, username);

   rs2 = ps2.executeQuery();
   if(rs2.next()) { // moving to the first row
       String result = rs2.getString(1); // i want this String to hold the first result
   } else {
       // throw Exception ?
   }
} catch (SQLException e) {
    e.printStackTrace();
}

Answer 4

您请求的列以columnIndex=0开头，但列索引以1开头

您可以使用我不久前使用的代码来查询整个表

public List<Article> findAll() {
    Connection connection = jdbConnectionWrapper.getConnection();
    List<Article> articles = new ArrayList<>();
    try {
        PreparedStatement preparedStatement = connection.prepareStatement("SELECT * FROM article");
        ResultSet resultSet = preparedStatement.executeQuery();

        while (resultSet.next()) {
            Article article = new Article();

            article.setId(resultSet.getLong(1));
            article.setName(resultSet.getString(2));
            article.setPrice(resultSet.getInt(3));

            articles.add(article);
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return articles;
}