Conditional UPDATES for INSERT ... ON DUPLICATE KEY

Inserting or updating records based on duplicate key conditions is a common scenario in database programming. However, sometimes you may need to further restrict the update condition, making it dependent on additional criteria. Unfortunately, the INSERT ... ON DUPLICATE KEY UPDATE syntax does not natively support WHERE clauses for conditional updates.

Overcoming the Limitation

To work around this limitation, you can utilize the IF() function within the UPDATE clause. The IF() function allows you to specify an alternative value based on a given logical expression.

Example Implementation

Consider the following INSERT ... ON DUPLICATE KEY UPDATE query:

INSERT INTO daily_events (created_on, last_event_id, last_event_created_at)
VALUES ('2010-01-19', 23, '2010-01-19 10:23:11')
ON DUPLICATE KEY UPDATE
  last_event_id = IF(last_event_created_at < VALUES(last_event_created_at), VALUES(last_event_id), last_event_id);

In this query, the UPDATE clause contains an IF() statement that checks whether the existing last_event_created_at value is less than the value being inserted. If the existing value is older, the update will be performed, replacing the last_event_id with the new value. Otherwise, the existing last_event_id will be preserved.

Additional Considerations

• Remember that IF() evaluates to NULL if the condition is false. Therefore, the updated value for last_event_id will also be NULL if the condition is not met.
• Consider using an additional WHERE clause or separate UPDATE query to ensure the desired filtering of the update.