Question

我有一个account_holder表，其中有一个userId字段，并且有一个数据集作为数据。我只想从account_holder中选择userId为data[3]的记录，即（数据集的4个字段）。我将查询写为：

Statement st = con.createStatement();

ResultSet rs = st.executeQuery("select * from account_holder where userID = 'data[3]' ");

但是我的rs是null，该条目未进入ResultSet对象rs。请告诉我使用executeQuery（）方法中的where子句编写sql查询的正确方法。

Answer 1

正如第一篇文章所述，这是处理此问题的一种方式：

     try {
        Statement statement = connection.createStatement(); // Creating Statement

        // Executing Statement
        ResultSet resultSet = statement.executeQuery("SELECT * FROM account_holder WHERE userID = '" + data[3] + "'");

        // Check if result set has entries
        if (resultSet.first()) // If so the user already exists
            System.out.println("User already exists! " + resultSet.getString("userID"));
        else // If not the user does not exist
            System.out.println("User does not exist!");

        // Clean up
        resultSet.close();
        statement.close();
        connection.close();
    } catch (SQLException e) {
        e.printStackTrace();
    }

但是根据您的情况，我实际上建议您使用PreparedStatement：

    try {
        // Making statement
        PreparedStatement preparedStatement = connection.prepareStatement("SELECT * FROM account_holder WHERE userID = ?");
        preparedStatement.setString(1, data[3]); // Set String for the first question mark in Query

        ResultSet resultSet = preparedStatement.executeQuery(); // Executing the Prepared Statement

        // Check if result set has entries
        if (resultSet.first()) // If so the user already exists
            System.out.println("User already exists! " + resultSet.getString("userID"));
        else // If not the user does not exist
            System.out.println("User does not exist!");

        // Clean up
        resultSet.close();
        preparedStatement.close();
        connection.close();
    } catch (SQLException e) {
        e.printStackTrace();
    }

Answer 2

您只需要在executeQuery方法中更改查询

示例：

Statement st = con.createStatement(); 
ResultSet rs = st.executeQuery("select * from account_holder where userID = '"+data[3]+"' ");

或者您也可以运行以下命令：

ResultSet rs = st.executeQuery("select * from account_holder where userID = "+data[3]+"");

如何在executeQuery方法中编写SQL查询？

2 个答案: