How Can MySQL Triggers Handle Duplicate Entries During INSERT Operations?

Aborting INSERT Operations in MySQL Triggers: A Comprehensive Guide

To prevent duplicate entries based on a combination of URL and parameter string, triggers can be employed. However, effectively handling duplicate entries requires a proper understanding of exception handling mechanisms.

Throwing Exceptions to C#

To raise an error that can be caught in C#, you can use the SIGNAL SQLSTATE statement within the trigger:

SIGNAL SQLSTATE '23000' SET MESSAGE_TEXT = 'Duplicate URL-Parameter combination detected.';

This statement triggers a SQL exception with error code '23000' and a custom error message.

Allowing Inserts

For scenarios where duplicate entries are acceptable, the trigger can simply terminate without raising an exception:

IF num_rows = 0 THEN
  -- Allow insert
ELSE
  -- Abort with an exception
END IF;

Improved Solution using SIGNAL

A more robust solution involves utilizing the SIGNAL statement, which provides a more flexible error handling mechanism:

CREATE TRIGGER `urls_check_duplicates`
BEFORE INSERT ON `urls`
FOR EACH ROW
BEGIN
  DECLARE msg VARCHAR(255);
  IF (num_rows > 0) THEN
    SET msg = 'Duplicate URL-Parameter combination detected.';
    SIGNAL SQLSTATE '23000' SET MESSAGE_TEXT = msg;
  END IF;
END

This solution provides a clearer error message and ensures compatibility with different database versions.

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