How to Implement Auto-Increment on a Secondary Primary Key Column in MySQL?

Achieving Auto-Increment on a Secondary Primary Key Column in MySQL

In situations where maintaining data integrity is crucial, using two columns as a primary key can be a suitable approach. However, challenges may arise when attempting to set auto-increment on one of the columns based on a specific value in the other.

For MySQL users, there are two options to consider when tackling this issue:

Using MyISAM Engine with AUTO_INCREMENT

For tables with the MyISAM engine, you can leverage the AUTO_INCREMENT attribute on a secondary column within a multiple-column index. The auto-increment value for the secondary column is calculated as follows:

MAX(auto_increment_column) + 1 WHERE prefix=given-prefix

This technique allows you to group data into ordered sets based on the value in the prefix column.

For Example:

Consider the following table named mytable with two primary key columns, database_id and table_id:

CREATE TABLE mytable (
  database_id MEDIUMINT NOT NULL,
  table_id MEDIUMINT NOT NULL AUTO_INCREMENT,
  other_column CHAR(30) NOT NULL,
  PRIMARY KEY (database_id,table_id)
) ENGINE=MyISAM;

Inserting data into mytable will automatically increment the table_id for each unique database_id value:

INSERT INTO mytable (database_id, other_column) VALUES
(1,'Foo'),(1,'Bar'),(2,'Baz'),(1,'Bam'),(2,'Zam'),(3,'Zoo');

When you retrieve data from the table, the rows will be ordered based on both the database_id and table_id columns:

SELECT * FROM mytable ORDER BY database_id,table_id;

+----------+-------------+--------------+
| table_id | database_id | other_column |
+----------+-------------+--------------+
|        1 |           1 | Foo          |
|        2 |           1 | Bar          |
|        3 |           1 | Bam          |
|        1 |           2 | Baz          |
|        2 |           2 | Zam          |
|        1 |           3 | Zoo          |
+----------+-------------+--------------+

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