How to abort INSERT operation in MySql trigger?

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开发者_开发知识库 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

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I came across this and although the solution works I later came across what feels to me like a better solution. I suspect this wasn't an option when this question was originally answered.

CREATE TRIGGER `TestTable_SomeTrigger`
BEFORE UPDATE ON `test_table`
FOR EACH ROW
BEGIN
    DECLARE msg VARCHAR(255);
    IF (SomeTestToFail = "FAIL!") THEN
        set msg = "DIE: You broke the rules... I will now Smite you, hold still...";
        SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = msg;
    END IF;

    -- Do any other code here you may want to occur if it's all OK or leave blank it will be
    --  skipped if the above if is true
END$$

This will now return a nice (or evil!) error message that you can trap. For more info on this see: http://dev.mysql.com/doc/refman/5.5/en/signal.html

I hope this helps someone else!

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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.

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If your table definition is for a NOT NULL value, you could set NEW to NULL, which would effectively not do the insert. You might have to turn strict sql mode on for this to work properly.