mysql 'after insert' trigger to calculate a field based on other fields

Question

I am trying to create a trigger that will update a GEOMETRY column based on lat/lng columns entered by the user. My trigger looks like so --

CREATE TRIGGER `tbl.foo`   
    AFTER INSERT ON `tbl` FOR EACH ROW  
    BEGIN  
        UPDATE tbl
        SET coord = Point(lng, lat)
        WHERE id = NEW.id; 
    END

However, I get the following error when I insert a new row with lng, lat values --

ERROR 1442 (HY000): Can't update table 'tbl' in stored function/trigger because it is already used by statement which invoked this stored function/trigger.

Can I not create such a trigger? If not, what is the way to automate this?

Solution

Try to use BEFORE INSERT trigger and modify value you need, e.g. -

CREATE TRIGGER trigger1
  BEFORE INSERT
  ON table1
  FOR EACH ROW
BEGIN
  SET NEW.column1 = 'another value';
END

---

EDIT

CREATE TABLE table_test_trigger (
  id INT(11) NOT NULL AUTO_INCREMENT,
  a INT(11) DEFAULT NULL,
  b INT(11) DEFAULT NULL,
  c INT(11) DEFAULT NULL,
  PRIMARY KEY (id)
);

DELIMITER $$

CREATE TRIGGER trigger1
    BEFORE INSERT
    ON table_test_trigger
    FOR EACH ROW
BEGIN
  SET NEW.c = NEW.a + NEW.b;
END
$$

DELIMITER ;

INSERT INTO table_test_trigger(a, b) VALUES (10, 5);

SELECT * FROM table_test_trigger;

+----+------+------+------+
| id | a    | b    | c    |
+----+------+------+------+
|  1 |   10 |    5 |   15 |
+----+------+------+------+

Comments

[punkish]

hmmm... how can I update the NEW.column1 with values from other columns unless those columns are populated first? See, the application allows users to enter lat/lng values. The Point() is calculated using those lat/lng values. UPDATE: actually, this suggestion does work. All I have to do is prefix all values with NEW. Thanks.