Question

I have a table with items in it (id, name, etc) and I want some kind of database scheme to be able to have multiple copies of the item while still being able to increment the ids. There will be a field called startdate or date_from_which_this_entry_should_be_used.

I’ve thought of two ways of implementing this:

1. Have a table with only ids (primary key, auto-increment) and do joins to a table that has all the item information.

   Advantages:

   • easy to understand
   • hard for someone that comes after me to get confused

   Disadvantages:

   • requires more debugging and coding since this system is already in use
   • seems weird to have a table with a single field

2. Have a single table using a sub-select to get the MAX value (+1) to apply to new items.

   Advantages:

   • single table
   • only minor code adjustments (but not all that different, maybe)

   Disadvantages:

   • prone to errors (manual increment, can’t allow deletions or the MAX value might be off)

Thanks in advance!

Answer 1

You should create a table called item_ids or something to generate id values. It’s okay that this has only a single column.

CREATE TABLE item_ids (
  item_id INT AUTO_INCREMENT PRIMARY KEY
) ENGINE=InnoDB;

You don’t even need to commit any data to it. You just use it to generate id values:

START TRANSACTION;
INSERT INTO item_ids DEFAULT VALUES;
SET @id = LAST_INSERT_ID();
ROLLBACK;

So now you have a concurrency-safe method to create new id’s.

Then you make a compound primary key for your items table. You must use MyISAM for this.

CREATE TABLE items (
  item_id INT,
  seq_id INT AUTO_INCREMENT,
  name VARCHAR(20),
  etc VARCHAR(20),
  PRIMARY KEY (item_id, seq_id)
) ENGINE=MyISAM;

MyISAM supports an auto-increment column in a compound primary key, which will start over at value 1 for each new item_id.* It also uses MAX(item_id)+1 so if you delete the last one, its value will be reallocated. This is unlike other use of AUTO_INCREMENT where a deleted value is not re-used.

Whether you insert a new item, or whether you insert a new copy of an existing item, you use a similar INSERT:

INSERT INTO items (item_id, name, etc) VALUES (@id, 'Stephane', 'etc');

The @id parameter is either a value of an existing item, or else the auto-generated value you got from the item_ids table.

* InnoDB supports auto-increment only as the first column of a primary or unique key, and it does not start over the count for each distinct value of the other column.