如何在 executeQuery 方法中编写 sql 查询？

Question

I have an account_holder table with a field userId and I have a data set as data.I want to select only that record from account_holder which having userId as data[3] ie (4 field of data set). I am writing query as:

Statement st = con.createStatement();

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

But my rs is null the entry not gets in to the ResultSet object rs. Plz tell me the correct way to write sql query with where clause inside the executeQuery() method.

Answer 1

As the first post said this is one way of handling this:

     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();
    }

But in your context I would actually recommend using a 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

You have to only change the query in the executeQuery method

example:

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

or you can also run the following command:

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