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:
Have a table with only ids (primary key, auto-increment) and do joins to a table that has all the item information.
Advantages:
Disadvantages:
Have a single table using a sub-select to get the MAX value (+1) to apply to new items.
Advantages:
Disadvantages:
Thanks in advance!
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.
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