MySQL insert only if not exist in two tables - php

Question

I have a little problem with MySQL query. I'm using PDO queries, which now just insert values into the tables (see below):
$stmt = $dbh -> prepare("INSERT INTO tmp (user_id, test_id, question_id, answer_id) VALUES (?, ?, ?, ?)");
$stmt -> bindParam(1, $_SESSION['UserID']); // binds parameter for user id
$stmt -> bindParam(2, $_GET['start_test']); // binds parameter for test id
$stmt -> bindParam(3, $_POST['question']); // binds parameter for selected answer
$stmt -> bindParam(4, $_POST['select_answer']); // binds parameter for selected answer
$stmt -> execute();
tmp table structure is the following:
CREATE TABLE IF NOT EXISTS `tmp` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`test_id` int(11) NOT NULL,
`user_id` int(11) NOT NULL,
`question_id` int(11) NOT NULL,
`answer_id` int(11) NOT NULL,
PRIMARY KEY (`id`)
)
I want to insert answer_id value if it's wasn't there and check if user_id at the same time, otherwise just update, example:
I have two users with user_id=1 and user_id=2, they're answering the questions and every answer for each student stores separately. User_id=1 picked answer_id = 3 and user_id=2 picked the answer_id = 3. However, then one user realised this was a wrong answer and wants to change it to another one (suppose answer_id=2), I want the query update the current answer_id=3 WHERE user_id=1 to answer_id=2

Answer 1

You can make an INSERT IGNORE for the same, which looks simpler and will be atomic with some issues.
Step are like
Create a unique index on (user_id,question_id, answer_id)
ALTER TABLE tmp add index user_qn_ans_idx (user_id,question_id, answer_id);
Now just do the insert, and probably on duplicate key update as well
INSERT INTO tmp (user_id,question_id, answer_id) values
(, , )
ON DUPLICATE KEY UPDATE answer_id = VALUES(answer_id)

Answer 2

Add a select count and do conditionals below:
$stmt = $dbh->prepare("SELECT count( 1 ) FROM tmp WHERE user_id = :user_id AND question_id = :question_id and answer_id = :answer_id");
$stmt->bindValue( 'user_id', 1 );
$stmt->bindValue( 'question_id', 1 );
$stmt->bindValue( 'answer', 1 );
$stmt->execute();
$oExists = $stmt ->fetch();
if( !empty( $oExists ) )
{
// Run update sql...
}
else
{
// Run insert sql...

Answer 3

$stmt = $dbh->prepare("SELECT count( 1 ) FROM tmp WHERE user_id = :user_id AND question_id = :question_id AND answer_id = :answer_id");
$stmt->bindValue( 'user_id', 1 );
$stmt->bindValue( 'question_id', 1 );
$stmt->bindValue( 'answer_id', 1 );
$stmt->execute();
$oExists = $stmt ->fetch();
if( !empty( $oExists ) ) {
echo "update";
$stmt = $dbh->prepare("UPDATE tmp SET answer_id = :answer_id WHERE user_id = {$_SESSION['UserID']} AND question_id = {$_POST['question']}"); //updates the counter each time test created
$stmt -> bindParam(':answer_id', $_POST['select_answer'], PDO::PARAM_INT);
$stmt -> execute();
} else {
echo "insert";
$stmt = $dbh -> prepare("INSERT INTO tmp (user_id, test_id, question_id, answer_id) VALUES (?, ?, ?, ?)");
$stmt -> bindParam(1, $_SESSION['UserID']); // binds parameter for user id
$stmt -> bindParam(2, $_GET['start_test']); // binds parameter for test id
$stmt -> bindParam(3, $_POST['question']); // binds parameter for selected answer
$stmt -> bindParam(4, $_POST['select_answer']); // binds parameter for selected answer
$stmt -> execute();
}
Here is always updates, but not inserts..