Question

I currently have two tables as follows:

data table

+-----+-------------+---------------+
| uri | field1      | field2        |
+-----+-------------+---------------+
|   1 | word        | somethingelse |
|   2 | somethinge  | values        |
|   3 | change_this | test          |
+-----+-------------+---------------+

log table

+--------+------+----------+-----------+-------------+---------------------+
| log_id | uri  | field_id | old_value | new_value   | modified_date       |
+--------+------+----------+-----------+-------------+---------------------+
|     14 | 3    | field1   | word32    | change_this | 2012-12-18 13:06:27 |
+--------+------+----------+-----------+-------------+---------------------+

When I update the value of field1 or field2 two, I get an entry in my ‘log’ table as to which field was changed, from what to what, when, and the uri of the record that was updated.

My question is regarding the trigger itself: Is it OK as is, or can this be accomplished more efficiently? I’ve honestly never written triggers before, and have pieced this together based on several snippets found through google. I welcome your thoughts/feedback.

DELIMITER $$
DROP TRIGGER IF EXISTS trig_update $$
CREATE TRIGGER trig_update AFTER UPDATE on data
FOR EACH ROW
BEGIN
DECLARE data_uri VARCHAR(32);
IF (NEW.field1 != OLD.field1) THEN
SET data_uri = (SELECT uri FROM data WHERE field1 = NEW.field1);
INSERT INTO log (uri, field_id, old_value, new_value, modified_date) 
VALUES (data_uri, "field1", OLD.field1, NEW.field1, NOW());
END IF; 
IF (NEW.field2 != OLD.field2) THEN
SET data_uri = (SELECT uri FROM data WHERE field2 = NEW.field2);
INSERT INTO log (uri, field_id, old_value, new_value, modified_date) 
VALUES (data_uri, "field2", OLD.field2, NEW.field2, NOW());
END IF; 
END$$
DELIMITER ;

Answer 1

You should eliminate redundant queries:

DELIMITER $$
DROP TRIGGER IF EXISTS trig_update $$
CREATE TRIGGER trig_update AFTER UPDATE on data
FOR EACH ROW
BEGIN
IF (NEW.field1 != OLD.field1) THEN
  INSERT INTO log (uri, field_id, old_value, new_value, modified_date) 
  VALUES (NEW.uri, "field1", OLD.field1, NEW.field1, NOW());
END IF; 
IF (NEW.field2 != OLD.field2) THEN
  INSERT INTO log (uri, field_id, old_value, new_value, modified_date) 
  VALUES (NEW.uri, "field2", OLD.field2, NEW.field2, NOW());
END IF; 
END$$
DELIMITER ;