Question

How can I insert a NULL value into a database field (MySQL) with char(2)?

Before I used mysql_query and I got in the database the following value: NU

Instead of this value there should be NULL.

After reading this question I switched to prepared statements. I still have NU in my database.

Now to my code. I’m reading out line for line of a CSV. Then I make

    if(empty($data[$c])) {
        $data[$c] = NULL;
    }

If I echo $data[1] I get NULL. Then I make my prepared query:

$stmt = $connection->prepare("INSERT INTO table (first, second, ..) VALUES (?, ?, ...)");
$stmt->bind_param("ss...", $data[0], $data[1], ...);
$res=$stmt->execute();

What is wrong?

Edit:

According to Shiplu I did the following:

Do it without giving the last params which causes NU:

$stmt = $connection->prepare("INSERT INTO employee (first, second, ...) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?)");
$stmt->bind_param("sssssssssssssii", $data[0], $data[1], ..., $last);

Using default:

$stmt = $connection->prepare("INSERT INTO employee (first, second, ..., last) VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, ?, default(last))");
$stmt->bind_param("sssssssssssssii", $data[0], $data[1], ...);

But still the same result. The char(2) field has default = NULL. How can that be? It seems that the problem is not with the query. I think the easiest solution would be an update query …

Create table:

CREATE TABLE IF NOT EXISTS `mytable` (
  `id` int(11) NOT NULL auto_increment,
  `text` varchar(128) default NULL,
  `no` int(11) default NULL,
  `problemfield` char(2) default NULL,
  PRIMARY KEY  (`id`),
  FULLTEXT KEY `text` (`text`)
) ENGINE=MyISAM  DEFAULT CHARSET=utf8 AUTO_INCREMENT=832 ;

Answer 1

I found the problem. I was reading out a csv file. This file was exported with phpmyadmin and it contained NULL strings. My check

if(empty($data[$c])) {
    $data[$c] = NULL;
}

wasn’t enough, because it never checked for the string NULL. So the solution is:

if(empty($data[$c]) || $data[$c]=='NULL') {
    $data[$c] = NULL;
}

So Andrew was right.