通过PHP比较Database中的列

I'm doing a program where I’m comparing in a database (Mysql-Workbench) 2 columns (difficulty, difficulty_student) of 2 different tables (EXERCISES, ANSWERS) in a column difficulty_choice of the table ANSWERS.

This is what I mean:

I’m comparing both tables using a VARCHAR (YES or NO). If the user has changed the difficulty of the exercise, the cell will be 'YES', if it has not been changed, the cell will be 'NO'.

These are my tables:

CREATE TABLE exercises (
    exercise_id INT,
    difficulty VARCHAR(30),
    PRIMARY KEY(exercise_id)
);

CREATE TABLE answers(
    exercise_id_fk INT,
    student_id INT,
    difficulty_change VARCHAR(3),
    difficulty_student VARCHAR(30),
    FOREIGN KEY(exercise_id_fk) REFERENCES exercises(exercise_id)
);

My problem is that the rows of the ANSWERS table don’t exist until the user presses the SUBMIT button in the program. So I have only managed to compare the columns that are in the table using the commands below in Mysql-Workbench.

What I need is to compare the columns in difficulty_change when the user presses SUBMIT. Can you help me do it? I can not get it.

I have managed to compare the columns using the following codes but I want them to be called from the program so that I do not have to go to Mysql-Workbench every time to execute them.

SELECT e.difficulty, a.difficulty_student, 
case when e.difficulty = a.difficulty_student then 'NO' else 'YES' 
         END  as difficulty_change
FROM exercises e 
INNER JOIN answers a  on e.exercise_id=a.exercise_id_fk;


UPDATE answers a 
INNER JOIN exercises e  on e.exercise_id=a.exercise_id_fk 
set a.difficulty_change = case 
     when e.difficulty = a.difficulty_student then 'NO' else 'YES' END
where e.exercise_id=a.exercise_id_fk;

This is my PHP, it might help:

<?php

if (isset($_POST['submit'])) {
    $user_id = $_SESSION['user_id'];
    $user_check_query = "SELECT * FROM users WHERE id='$user_id'";
    if(isset($_POST['choice'], $_POST['choose'])){
            $choice_answer=$_POST['choice'];
            $difficulty=$_POST['choose'];
//      */$user_id = $_SESSION['user_id'];*/
            $query = "INSERT INTO answers (exercise_id_fk, student_id, difficulty_student, choice_answer) VALUES ('$id','$user_id', '$difficulty', '$choice_answer')";
            $sql=mysqli_query($conn,$query);

    }
}
?>

Try:

$query = "INSERT INTO answers (exercise_id_fk, student_id, difficulty_change, difficulty_student, choice_answer) VALUES ('$id','$user_id', (SELECT IF(difficulty='$difficulty','NO','YES') FROM exercises WHERE exercise_id=$id), '$difficulty', '$choice_answer')";

Use your submit step to only update the db. Add code to query the db again to retrieve the data. Add a line to call your display page again (your display page calls itself) to display your updated table.