How Can I Prevent Auto-Increment Gaps When Inserting Duplicate Rows in MySQL?

Avoiding Auto-Increment on MySQL Duplicate Inserts

Inserting duplicate rows into a table can result in unexpected behavior with MySQL's auto-increment mechanism. In MySQL 5.1.49, the auto-increment ID continues to increment even if an INSERT IGNORE statement is used. This can lead to undesirable gaps in the ID sequence.

To resolve this issue, you can employ the following strategies:

1. SELECT Before Insert: Before attempting an insert, execute a SELECT statement to check if the duplicate row already exists. If it does, the insert can be skipped. However, this approach can be inefficient for large tables.
2. INSERT with Subquery: Instead of INSERT IGNORE, use this modified INSERT statement:

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

This statement first checks if the tag already exists in the table. If it does not, the row is inserted without triggering the auto-increment mechanism. The subquery ensures that the insert only occurs once.

3. Composite Unique Key: Consider creating a composite unique key on multiple columns. This ensures that duplicate rows are never inserted, eliminating the need to manually check for duplicates.

It's important to note that these solutions may not be suitable for all scenarios. If performance is a concern, it may be necessary to evaluate other options, such as using a separate table to store unique values and referencing them in your main table.

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