How to Perform Conditional Updates in MySQL\'s INSERT ... ON DUPLICATE KEY UPDATE?

Conditional Update in INSERT ... ON DUPLICATE KEY UPDATE

In MySQL, the INSERT ... ON DUPLICATE KEY UPDATE syntax allows for automatic update of table data when a duplicate key is encountered during an insert operation. However, the UPDATE portion of the query does not support the use of a WHERE clause for specifying conditional updates.

Workaround Using IF()

To overcome this limitation, one can utilize the IF() function to implement conditional updates within the ON DUPLICATE KEY UPDATE clause. The IF() function evaluates a condition and returns a different value based on the result.

For instance, consider the following INSERT ... ON DUPLICATE KEY UPDATE statement:

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 statement, the IF() function checks whether the value of the last_event_created_at column in the database table is less than the value being inserted (VALUES(last_event_created_at)). If true, it updates the last_event_id column with the new value from the insert statement. Otherwise, it leaves the existing value unchanged.

This workaround allows for conditional updates in INSERT ... ON DUPLICATE KEY UPDATE statements without resorting to combinations of INSERT/UPDATE/SELECT queries, which may not be suitable for replication scenarios.

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