ERROR 1442: Can't update table in stored function/trigger because it is already used by statement which invoked this stored function/trigger

I have a table that I want : when the table has been updated, 2 fields of that (title and description) change and take value from another table

This is my trigger:

drop trigger trigger_trade_request ;
CREATE TRIGGER trigger_trade_request AFTER UPDATE ON `trade_request` 
FOR EACH ROW 
BEGIN 

  IF NEW.title = null THEN

    UPDATE `trade_request_type`,`trade_request` 
       SET NEW.title = `trade_request_type`.title ,
           NEW.description = `trade_request_type`.description
     WHERE `trade_request_type`.id = NEW.trade_request_typeId;

  END IF开发者_如何学Python;   
END;

My tables:

• trade_request_type
• trade_request

error:

ERROR 1442 : Can't update table 'trade_request' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

It does not work! What is the prob?

1. Use is null to check for null values.
2. To change the value of the updated row use BEFORE trigger and SET on the NEW row.

Try this:

CREATE TRIGGER trigger_update_request BEFORE UPDATE ON `trade_request`
FOR EACH ROW
BEGIN
    IF NEW.title IS NULL THEN
        SET NEW.title = (SELECT title FROM trade_request_type
            WHERE trade_request_type.id = NEW.trade_request_typeId);
        SET NEW.description = (SELECT description FROM trade_request_type
            WHERE trade_request_type.id = NEW.trade_request_typeId);
    END IF;
END;

Probably you can use the SELECT...INTO syntax instead of SET, like so:

CREATE TRIGGER trigger_update_request BEFORE UPDATE ON `trade_request`
FOR EACH ROW
BEGIN
    IF NEW.title IS NULL THEN
        SELECT title, description
        FROM trade_request_type
        WHERE trade_request_type.id = NEW.trade_request_typeId
        INTO NEW.tile, NEW.description;
    END IF;
END;

This is because you are trying to update the other table too. Try that:

UPDATE `trade_request`
   SET NEW.title = `trade_request_type`.title ,
       NEW.description = `trade_request_type`.description
 FROM `trade_request_type`
 WHERE `trade_request_type`.id = NEW.trade_request_typeId;
   AND `trade_request_type`.id = `trade_request`.id

Three problems:

1. To test for null, use is null (not = null which is never true)
2. To set new values in your incoming row, use NEW.column = some_value
3. You can't update a table twice in one trigger

Try this:

drop trigger trigger_trade_request ;
CREATE TRIGGER trigger_trade_request BEFORE UPDATE ON `trade_request` 
FOR EACH ROW 
BEGIN 
  IF NEW.title is null THEN
       SET NEW.title = (select title from trade_request_type WHERE trade_request_type.id = NEW.trade_request_typeId);
       SET NEW.description = (select description from trade_request_type WHERE trade_request_type.id = NEW.trade_request_typeId);
  END IF;   
END;