Can Auto-Increment Columns Exist Without Being the Primary Key in MySQL?

Addressing Incorrect Table Definition Error: Balancing Auto-Increment and Primary Key

The scenario involves a MySQL table where the id column serves as an auto-increment field for visual convenience, while the memberid column acts as the actual unique key. However, the attempt to define the table with PRIMARY KEY (memberid) results in an error (1075) stating there can only be one auto column and it must be a key.

Resolving the Error: Preserving Auto-Increment and Unique Key

To resolve the issue, it is possible to have an auto-incrementing column that is not the PRIMARY KEY, provided an index (key) is defined on it. Here's the modified table definition:

<code class="sql">CREATE TABLE members (
  id int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
  memberid VARCHAR(30) NOT NULL,
  `time` TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
  firstname VARCHAR(50) NULL,
  lastname VARCHAR(50) NULL,
  PRIMARY KEY (memberid),
  KEY (id)                          # or: UNIQUE KEY (id)
) ENGINE = MYISAM;</code>

By adding a KEY or UNIQUE KEY index on the id column, the auto-increment functionality is maintained while the memberid column becomes the primary key, allowing efficient queries based on the memberid value.

Choosing the Optimal Approach: Balancing Performance and Space

The best choice depends on the relative importance of performance and disk space. If performance is paramount, maintaining the auto-incrementing id column and using an index on memberid offers a balance:

• The auto-incrementing id provides efficient sorting and range queries.
• The index on memberid ensures quick lookups using the unique member identifier.

However, if disk space is a significant concern, consider removing the id column altogether and relying on the memberid column as both the primary key and auto-incrementing field. This approach sacrifices some performance for improved space utilization. Ultimately, the choice between performance and space depends on the specific requirements of the application.

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