How to Implement Auto-Increment by Group in MySQL?

Auto-Increment by Group in MySQL

MySQL provides a convenient way to automatically increment a field value based on a grouping column. This is especially useful in scenarios where records need to be grouped and numbered within each group.

For MyISAM and BDB Tables

For MyISAM and BDB tables, auto-increment can be defined as a secondary key part. For example, consider the following table structure:

CREATE TABLE foo (
   id          INT AUTO_INCREMENT NOT NULL,
   group_field INT NOT NULL,
   name        VARCHAR(128),

   PRIMARY KEY(group_field, id)
);

In this structure, the id field is defined as auto-increment, but it is also part of the primary key along with the group_field. According to the MySQL documentation:

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.

Example

Using the foo table created above, let's insert some sample data:

INSERT INTO foo (group_field, name) VALUES (1, 'test'), (1, 'test2'), (2, 'test3'), (2, 'test4'), (3, 'test5'), (3, 'test6');

After inserting the data, the contents of the foo table will look like:

+----+-----------+------+
| id | group_field | name |
+----+-----------+------+
| 1  |          1 | test |
| 2  |          1 | test2 |
| 3  |          2 | test3 |
| 4  |          2 | test4 |
| 5  |          3 | test5 |
| 6  |          3 | test6 |
+----+-----------+------+

As you can see, the id column has been automatically incremented within each group, based on the group_field value.

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