Insert new row in a table and auto id number - php

Question

I want to insert a new row in my table. I want the id to be generated right automatically and not asked from the user. The user only provides title and text. I wrote this code in PHP:
<?php
$hostname = "localhost";
$database = "mydb";
$username = "myuser";
$password = "mypsw";
$link = mysql_connect( $hostname , $username , $password ) or
die("Attention! Problem with the connection : " . mysql_error());
if (!$link)
{
die('Could not connect: ' . mysql_error());
}
mysql_query("SET NAMES ‘utf8’",$link);
mysql_select_db("mydb", $link);
$lastid=mysql_insert_id();
$lastid=$lastid+1;
$sql="INSERT INTO announcements VALUES ('$lastid',CURDATE(),'$_POST[title]','$_POST[text]')";
if (!mysql_query($sql,$link))
{
die('Error: ' . mysql_error());
}
mysql_close($link);
header("Location: announcement.php");
?>
Sadly when I test it on my website, I get this error:
Error: Duplicate entry '0' for key 'PRIMARY'
Is mysql_insert_id() not working? What is wrong?

Answer 1

Don't do this. mysql will happily create an auto_increment column for you:
CREATE TABLE x (
id int not null primary key auto_increment
^^^^^^^^^^^^^^---add this to your PK field
);
INSERT INTO x (id) VALUES (null); // creates id = 1
INSERT INTO x (id) VALUES (null); // creates id = 2
mysql_insert_id() only returns the last id created by the CURRENT connection. You haven't inserted any data yet when you first run it, so you get back nothing.
Your version is incredibly vulnerable to race conditions. There is NO guarantee that the last ID you retrieve with mysql_insert_id() will not ALSO get retrieved by another copy of the script running in parallel, and get sniped out from under this copy of the script.

Answer 2

The primary key column on announcements should be auto_increment. When you do mysql_insert_id() it retrieves the id from the last query executed from that connection.
Because the INSERT is the query you are currently performing, it errors.
Try
INSERT INTO announcements
(date_field, title, text)
VALUES (CURDATE(),'$_POST[title]','$_POST[text]')
Just replace 'date_field', 'title', and 'text' with the applicable column names.
Alternatively the following should also work, as a NULL value in the AutoIncrement value should be acceptable
INSERT INTO announcements VALUES (NULL,CURDATE(),'$_POST[title]','$_POST[text]')
As mentioned in the other suggestion posted, you should make sure that the primary key field of the announcements table is set to be auto_increment.
Just for completion, you would use mysql_insert_id() when you want to use the id for the row you just inserted, i.e. if you then want to select the row you just inserted you could do
'SELECT * FROM announcements WHERE id = '.mysql_insert_id()

Answer 3

The problem is that you are asking for last insert id and you didn't inserted anything.
Convert your ID field in db to be autoincrement if its not.
Insert into database your announcment
Then ask for id using mysql_insert_id to get it.
But I see that you are not using it only when inserting then you don't need that functionality anyhow. Just insert without ID like this
"insert into announcements (InsertDate, Title, Text) VALUES (CURDATE(), '$_POST[title]', '$_POST[text]')";
and you should really be careful with your queries when using values from $_POST or $_GET or any other user typed value. There is possibility to execute SQLInjection through your form fields, so I suggest you to use mysql escape command or use parameters.
I hope this helps.

Answer 4

Assuming your table is set up properly, with the id field as AUTO_INCREMENT, you just need to perform an INSERT where you do not specify a value for id. That means you must specify the names of the columns you are inserting. So this line:
$sql="INSERT INTO announcements VALUES ('$lastid',CURDATE(),'$_POST[title]','$_POST[text]')";
becomes this
$sql="INSERT INTO announcements (`date`,`title`,`text`) VALUES (CURDATE(),'$_POST[title]','$_POST[text]')";
I guessed what your column names might be. Obviously they need to match your table definition.
If you do this, then the mysql_insert_id() function will return the id of the row you just inserted. (That is, it gives you the value of the previous insert, not the next one.)

Answer 5

You probably want to add "auto increment" to the table when creating it.
This will add an id automatically when inserting something.
e.g.
CREATE TABLE announcements
(
id int NOT NULL AUTO_INCREMENT,
PRIMARY KEY(id),
some_date int(11),
title varchar(200),
text varchar(3000)
);
mysql_insert_id "Retrieves the ID generated for an AUTO_INCREMENT column by the previous query " - http://php.net/manual/en/function.mysql-insert-id.php