creating primary key based on date

Question

I am trying to create a unique ID for each record based on the date the record is created. Example, if the record is created today, I want the key to be 20101130XX where XX

Answer 1

This approach simply won't work because @Charles is true. You can't rely on a timestamp because there's a chance of duplicates (collision).

Create a simple autoincrement id then save the timestamp too and you'll be happy.

Answer 2

From dev.mysql.com: example-auto-increment.html

For MyISAM and BDB tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1 WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups.

So, make 2 columns on the table, one dateEntered and one (auto_incremented) id, like this:

CREATE TABLE yourTable (
    dateEntered DATE NOT NULL,
    id INT NOT NULL AUTO_INCREMENT,
    name CHAR(30) NOT NULL,
    PRIMARY KEY (dateEntered, id)
) ENGINE=MyISAM;

If you don't use ISAM but InnoDB, I think you'll have to write your own trigger that implements this behaviour.

Answer 3

What will you do if the number of entries per day exceeds 99? If you strongly want to use "20101130XX" index, I would recommend you use a compound key:

CREATE TABLE `test` (
  `date` date NOT NULL,
  `id` tinyint(4) NOT NULL,
  `other_fields` varchar(255) DEFAULT NULL,
  PRIMARY KEY (`date`,`id`)
);

Answer 4

Just use microtime() to be sure that there are no duplicate entries.

function getUniqueID() {
  $mt = explode(' ', microtime());
  return $mt[1] . substr($mt[0], 2, 7);
  }

P.S. You can always use lower precision then 7 digits.