Updating table in trigger after update on the same table

问题

How can I update table's column in a trigger after update on the same table?
Here's the trigger:

CREATE TRIGGER upd_total_votes AFTER UPDATE ON products_score
FOR EACH ROW
    UPDATE
        products_score 
    SET
        products_score.votes_total =
            (SELECT
                 (votes_1 + votes_2 + votes_3 + votes_4 + votes_5)
             FROM
                 products_score
             WHERE
                 id = new.id)

Now when I update the table like

UPDATE products_score SET votes_1 = 5 WHERE id = 0

this doesn't work, as I get the following:

#1442 - Can't update table 'products_score' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

So how on earth I can get this to work?

回答1:

If you change your trigger to BEFORE instead of AFTER you could do it like this:

CREATE TRIGGER upd_total_votes BEFORE UPDATE ON products_score 
FOR EACH ROW 
BEGIN
    SET new.votes_total = new.votes_1 + new.votes_2 + new.votes_3 + new.votes_4 + new.votes_5 
END
;

回答2:

You can't have this the way you have setup because a trigger can't query other rows of the same table that it is defined on. Istead you can use a Before Update Trigger toachieve what you want:

CREATE TRIGGER upd_total_votes BEFORE UPDATE ON products_score FOR EACH ROW     
BEGIN
    SET NEW.votes_total = NEW.votes_1 + NEW.votes_2 + NEW.votes_3 + NEW.votes_4 + NEW.votes_5;
END;

Or use a stored procedure to update the table.

来源：https://stackoverflow.com/questions/4547465/updating-table-in-trigger-after-update-on-the-same-table