Why Does MySQL's Auto-Increment Misbehave with ON DUPLICATE KEY UPDATE?

ON DUPLICATE KEY Auto-Increment Anomaly

In MySQL, using ON DUPLICATE KEY UPDATE when inserting data into a table can lead to unexpected behavior with auto-incrementing columns. As described in the question, it is observed that the auto-increment value increments as expected upon the first insertion, however, when the ON DUPLICATE KEY condition is triggered and a new row is inserted, the auto-increment value appears irregular.

The answer to this issue lies in the way MySQL handles ON DUPLICATE KEY UPDATE. As documented in the MySQL documentation, during this operation, MySQL performs an update on the existing row rather than inserting a new one. In the case of auto-increment columns, the INSERT statement increments the auto-increment value before the update is applied. However, the UPDATE statement does not increment the value.

Here's a simplified explanation:

• INSERT attempts to insert the new row, incrementing the auto-increment value.
• The ON DUPLICATE KEY condition is detected, triggering the update.
• The update sets the values in the existing row, including the auto-increment column, to the values specified in the update clause.

As a result, the auto-increment column in the updated row does not reflect the expected value that would have been assigned if a new row had been inserted. It is worth noting that relying on auto-increment columns to have no gaps is not recommended. If strict sequential numbering is crucial, it is advisable to implement custom logic using triggers or calculated values on output to maintain the desired incrementing behavior.

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