How Can I Use MySQL Triggers to Enforce Uniqueness on URL-Parameter Pairs and Handle Duplicate Entries in C#?

Aborting INSERT Operations in MySQL Triggers: Achieving Unique Constraints on URL-Parameter Pairs

To enforce uniqueness on the combination of URL and parameter string in a MySQL table, triggers can be employed. However, aborting insert operations in a trigger to prevent duplicate entries requires specific handling. This article guides you through the steps to throw exceptions and conditionally allow inserts in your trigger.

Throwing Exceptions to C#

To signal a duplicate entry error to the C# code, you can utilize MySQL's SIGNAL statement within the trigger. Below is the updated code snippet for the urls_check_duplicates trigger:

CREATE TRIGGER urls_check_duplicates
BEFORE INSERT ON urls
FOR EACH ROW

BEGIN
DECLARE num_rows INTEGER;

SELECT COUNT(*)
INTO num_rows
FROM urls
WHERE url = NEW.url AND params = NEW.params;

IF num_rows > 0 THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Duplicate URL-parameter pair found';
ELSE
    ... Allow insert ...
END

In your C# code, you can handle the error thrown by the trigger using the MySqlException class:

try
{
    // Execute INSERT statement
}
catch (MySqlException ex)
{
    if (ex.Message.Contains("Duplicate URL-parameter pair found"))
    {
        // Handle duplicate entry error
    }
}

Conditional Insert Allowance

To allow insertion only if there is no duplicate entry, simply remove the ELSE block in the trigger code. The following code inserts only unique URL-parameter pairs:

CREATE TRIGGER urls_check_duplicates
BEFORE INSERT ON urls
FOR EACH ROW

BEGIN
DECLARE num_rows INTEGER;

SELECT COUNT(*)
INTO num_rows
FROM urls
WHERE url = NEW.url AND params = NEW.params;

IF num_rows > 0 THEN
    SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'Duplicate URL-parameter pair found';
END

By using a combination of SIGNAL and conditional execution in the trigger, you can effectively enforce the unique constraint on URL-parameter pairs in your MySQL table.

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