Why Does MySQL's AUTO_INCREMENT Behave Inconsistently with INSERT ... ON DUPLICATE KEY UPDATE?

ON DUPLICATE KEY AUTO INCREMENT Issue in MySQL

Problem:

When using the INSERT ... ON DUPLICATE KEY UPDATE statement with a table containing an auto-incrementing column (id), the id value appears to increment oddly when a duplicate key is encountered. During the initial insert (without duplicate key), the id increments normally. However, when the ON DUPLICATE KEY clause triggers and a new row is inserted, the id value becomes inconsistent.

Answer:

This behavior is intentional and documented by MySQL. When a row is inserted that would cause a duplicate value in a unique index or primary key, MySQL performs an update of the old row instead of inserting a new one. This means that even though the INSERT statement is attempted first, the id value is already incremented at that stage.

The update that follows does not increment the id value because auto-increment columns are not updated by the UPDATE statement. Instead, the id value from the original (now updated) row is preserved.

Solution:

To maintain auto-increment without gaps, it's necessary to use a different approach. One option is to calculate incremental values on the output rather than relying on the auto-increment column. Alternatively, a more complex approach involving table locks and triggers can be used to renumber rows and ensure no gaps in the id values.

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