Why Does MySQL's Auto-Increment Behavior Change with `ON DUPLICATE KEY UPDATE`?

Auto Increment Anomalies with ON DUPLICATE KEY

In MySQL, when using the INSERT ... ON DUPLICATE KEY UPDATE statement, unexpected behavior can arise with auto increment columns. This issue stems from the fact that MySQL attempts to perform the insert first, causing the id to be incremented. Subsequently, when the duplicate is detected, the update occurs without affecting the id value.

Explanation:

According to MySQL documentation, if a duplicate value is detected in a unique index or primary key during an insert with ON DUPLICATE KEY UPDATE, MySQL executes an update on the old row. While this behavior is similar to an UPDATE statement, there is a crucial difference when dealing with auto-increment columns in InnoDB tables.

In such cases, the INSERT statement increments the auto-increment value, while the subsequent UPDATE does not. This results in a discrepancy between the auto-increment value and the actual number of rows in the table.

Solution:

It's important to acknowledge that auto-increment sequences may not always be contiguous. If gaps in sequence are unacceptable, a more complex solution is required, typically involving table locking and triggers to recalculate incremental values. However, for most scenarios, it is not advisable to rely on unbroken auto-increment sequences.

Instead, consider calculating incremental values on the output side to ensure consistency. This approach eliminates the reliance on auto-increment columns and allows for more efficient inserts and updates.

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