How to += string with sql query using php? - php

Question

Uncaught mysqli_sql_exception: Truncated incorrect DOUBLE value: '0-' in ... Stack trace: #0 ...(68): mysqli->query('UPDATE `Results...') #1 {main} thrown in ... on line 68
That's the error I am getting with the ellipses representing the file pathway.
The php I have:
for($i = 1; $i < $maxQuestions; $i++){
$answer = $_POST["question$i"] == "Yes" ? "1-" : "0-";
$connection->query("UPDATE `Results`
SET `question$i` = `question$i` + '$answer'
WHERE `ID` = '$id'");
}
The problem is with the SET `question$i` = `question$i` + '$answer' as I am trying to basically keep a history of the question's answers. For example, I may want the stored data to go from "0-" to "0-1-". How do you += a string with php/sql?
EDIT:
I have tried SET `question$i` = concat(`question$i` + '$answer') and I am getting the same error.

Answer 1

I changed it to
for($i = 1; $i < $maxQuestions; $i++){
$answer = $_POST["question$i"] == "yes" ? "1-" : "0-";
$qColumn = "question" . strval($i);
$questionData = $data["question$i"] . $answer;
$connection->query("UPDATE `Results`
SET `$qColumn` = '$questionData'
WHERE `ID` = '$id'");
}
This worked, but I'm still not sure why my initial code did not work. If someone could post a better answer/solution and why my initial code didn't work, please do. Thanks!

Answer 2

In mysql You can do it dierctly using concat this way
SET `question$i` =concat(`question$i`, '$answer')

Answer 3

You should not have column names ending with numbers because this is an indicator that your schema is not correct. You should normalise the database and have questions as a separate tables joined to your results table. Your results should not be concatenated into a single column either. Have a separate row for each result.
However, if you want to achieve this with your current set-up you could use CONCAT(). The query should look similar to this:
$types = '';
$setString = [];
$values = [];
for($i = 1; $i < $maxQuestions; $i++){
// concat types
$types .= 's';
// gather values
$values[] = isset($_POST["question$i"]) ? "1-" : "0-";
// build SET statement
$columnName = "question" . $i;
$setString[] = $columnName.' = CONCAT('.$columnName.', ?)';
}
$sql = "UPDATE Results SET ".implode(', ', $setString).' WHERE ID = ?';
$stmt = $connection->prepare($sql);
$stmt->bind_param($types, ...$values);
$stmt->execute();