How Can I Prevent MySQL Auto-Increment on Duplicate Inserts?

Preventing Auto Increment on MySQL Duplicate Inserts

When using MySQL 5.1.49, inserting duplicate rows into an InnoDB table with an auto-incrementing primary key can result in incrementing the ID field even if the insert is ignored. This can lead to unwanted gaps in the ID sequence.

Solution

To prevent auto increment on duplicate insertions, you can use a modified INSERT statement:

INSERT INTO tablename (tag)
SELECT $tag
FROM tablename
WHERE NOT EXISTS(
    SELECT tag
    FROM tablename
    WHERE tag = $tag
)
LIMIT 1

Where $tag is the tag value you want to insert. This approach performs a check to see if the tag already exists before attempting an insert. If the tag doesn't exist, the insert is performed and the auto increment counter is not affected.

Advantages of this Approach:

• Prevents auto increment on duplicates: It ensures that the auto increment counter is only incremented when a new tag is inserted.
• Fast: The EXISTS check is executed efficiently if the table is properly indexed.
• Handles the first tag case: You can seed the table with a placeholder tag or perform a separate check for an empty table to handle the first tag insertion.

By using this modified INSERT statement, you can prevent the auto increment counter from incrementing on duplicate inserts, ensuring a sequential and gap-free ID sequence.

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