How to abort INSERT operation in MySql trigger?

Question

Hi all. I have a table containing an url and a string representing its parameters. The problem is I want an url and a parameterstring to be the unique constraint for the table - aka no entries can have the same url AND parameter string. The parameter string can be of arbitrary length (longer than 800bytes or so which is the max length for a MySql key, so I cant use Unique(url, params) since it throws an error...).

I thought about using triggers to do this, but how do I throw an exception/raise an error if the trigger discovers the insert is about to insert a duplicate entry? I imagine I would like to have a MySqlException thrown like MySql does with duplicate primary keys etc so I can catch it in my C# code.

I have two pieces in the trigger I need to get help with: ... Abort throw exception to C# ... How do I throw an exception etc to C#? ... Allow insert ... - how do I just allow the insert if there is no duplicate entry?

Heres the trigger code:

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
   ... ABORT/throw exception to C# ...
ELSE
   ... Allow insert ...
END

Answer 1

The comments in the mysql documentation about triggers suggest that there is no such feature in mysql. The best you can do is to create a separate table for your trigger errors, as suggested on the same page.