Column NOT NULL but script is still creating a row - php

Question

I have a script, which I use to INSERT data to my database. In HTML I have multiple textboxes. If data is entered into the textbox my script is running the INSERT statement. My script is also running the INSERT statement if there is no data entered into the textbox.
If there is no data entered into the textbox, the script creates an empty row in the database. I have tried to block this by changing the column to NOT NULL.
But it seems to be not working.
Does someone know the reason for that and how I can solve it?
Edit 1:
$sql1 = "SELECT * FROM product WHERE id='" . $id1 . "' AND user_id='" . $_SESSION['USER_ID'] . "'";
$result1 = $link->query($sql1);
if ($result1->num_rows > 0) {
$sql11 = "UPDATE product SET voorraad = quantity -" . $quantity . " WHERE user_id='" . $_SESSION['USER_ID'] . "' AND id='" . $id1 . "'";
if(mysqli_query($link, $sql11)){
echo '<p>product1 is updated</p><br />';
} else {
echo '<p>error</p>';
}
} else {
$query1 = "INSERT INTO product(user_id, name, price, tax) VALUES (?, ?, ?, ?)";
$stmt = $db->prepare($query1);
$exec1 = $stmt->execute(array($_SESSION['USER_ID'], $name1, $price1, $tax1));
if($exec1){
echo '<p>product1 is created</p>';
} else {
echo '<p>error</p>';
}
}

Answer 1

You are inserting empty strings, and empty string are not NULL, to check for NULL error do:
INSERT INTO addresses (street, city, state, zip) VALUES (NULL, NULL, NULL, NULL);
and you will see error. The NOT NULL checks only for values that are not NULL.
To prevent empty string either you have to use triggers, or do the checks on server side programming language to convert empty strings to NULL before performing INSERT query. An example trigger for INSERT may be like: (this is just an example)
CREATE TRIGGER avoid_empty
BEFORE INSERT ON addresses
FOR EACH ROW
BEGIN
IF street = '' THEN SET street = NULL END IF;
END;

Answer 2

you can verify variables and execute query if variable is not null & empty ,else write values are null, must pass data