Question

error 1093: MySQL can’t specify target table 'SENTIERO' for update in FROM clause

This is my trigger:

CREATE TRIGGER lunghezza_sentiero_datoderivato_INSERT
AFTER INSERT ON SENTIERO_HA_TAPPA
FOR EACH ROW

BEGIN

UPDATE SENTIERO
SET lunghezza= (SELECT SUM(lunghezza)
                FROM TAPPA, SENTIERO as S2, SENTIERO_HA_TAPPA
                WHERE NEW.IDsentiero=S2.IDsentiero
                and SENTIERO_HA_TAPPA.IDtappa=TAPPA.IDtappa);
WHERE IDsentiero IN (SELECT IDsentiero
                     FROM TAPPA, SENTIERO, SENTIERO_HA_TAPPA
                     WHERE SENTIERO_HA_TAPPA.IDsentiero=SENTIERO.IDsentiero
                     and NEW.IDtappa=SENTIERO_HA_TAPPA.IDtappa);
END$$

I’ve found this article about this issue, check it on
http://verysimple.com/2011/03/30/mysql-cant-specify-target-table-for-update-in-from-clause/

Is it the only chance i have? Please help me

EDIT:: i’ve just added the ‘WHERE’ clause but it signals me “error syntax”… why?

Answer 1

Try this instead:

UPDATE SENTIERO s
INNER JOIN
(
   SELECT IDsentiero, TheSum
   FROM
   (
      SELECT S2.IDsentiero, SUM(s2.lunghezza) AS TheSum
      FROM SENTIERO AS S2 
      INNER JOIN SENTIERO_HA_TAPPA AS st ON st.IDsentiero = S2.IDsentiero
      INNER JOIN TAPPA             AS  t ON st.IDtappa    = t.IDtappa
      GROUP BY S2.IDsentiero
   ) AS Sub
) AS s2 ON s.IDsentiero  = s2.IDsentiero 
SET s.lunghezza = s2.TheSum
WHERE S2.IDsentiero = NEW.IDsentiero;

---

Update:

You have to get the sum of the values of lunghezza from the TAPPA table not from the SENTIERO, thats why you were getting NULL values. So the final CREATE TRIGGER code should be like this:

CREATE TRIGGER lunghezza_sentiero_datoderivato_INSERT
AFTER INSERT ON SENTIERO_HA_TAPPA
FOR EACH ROW

BEGIN

  UPDATE SENTIERO s
  INNER JOIN
  (
     SELECT IDsentiero, TheSum
     FROM
     (
        SELECT S2.IDsentiero, SUM(t.lunghezza) AS TheSum
        FROM SENTIERO AS S2 
        INNER JOIN SENTIERO_HA_TAPPA AS st ON st.IDsentiero = S2.IDsentiero
        INNER JOIN TAPPA             AS  t ON st.IDtappa    = t.IDtappa
        GROUP BY S2.IDsentiero
     ) AS Sub
  ) AS s2 ON s.IDsentiero  = s2.IDsentiero 
  SET s.lunghezza = s2.TheSum;

END

SQL Fiddle Demo

Note that: This trigger will update the values of lunghezza in the table SENTIERO, when any row being inserted into the table SENTIERO_HA_TAPPA for all the IDsentiero in the SENTIERO, not just the value of the new inserted IDsentiero.

To update only the value of lunghezza for the new inserted value of IDsentiero into the table SENTIERO_HA_TAPPA only, add a WHERE S2.IDsentiero = NEW.IDsentiero to the UPDATE statement of the trigger. Like this:

CREATE TRIGGER lunghezza_sentiero_datoderivato_INSERT
AFTER INSERT ON SENTIERO_HA_TAPPA
FOR EACH ROW

BEGIN

  UPDATE SENTIERO s
  INNER JOIN
  (
     SELECT IDsentiero, TheSum
     FROM
     (
        SELECT S2.IDsentiero, SUM(t.lunghezza) AS TheSum
        FROM SENTIERO AS S2 
        INNER JOIN SENTIERO_HA_TAPPA AS st ON st.IDsentiero = S2.IDsentiero
        INNER JOIN TAPPA             AS  t ON st.IDtappa    = t.IDtappa
        GROUP BY S2.IDsentiero
     ) AS Sub
  ) AS s2 ON s.IDsentiero  = s2.IDsentiero 
  SET s.lunghezza = s2.TheSum
  WHERE S2.IDsentiero = NEW.IDsentiero;

END;

For instance, if you create the three tables, then insert the data to the tables and then create that trigger. Then do an insert into the table SENTIERO_HA_TAPPA like this:

INSERT INTO `SENTIERO_HA_TAPPA` (`IDtappa`, `IDsentiero`) VALUES (14, 4); 

Then the trigger will update the value of the IDsentiero = 4 only in the table SENTIERO, not all the values of it. And the values of lunghezza for other IDsentiero‘s will be NULLs:

• Updated SQL Fiddle Demo.

So, you have to create your tables and the triggers before any insert. Then do the insertions into the tables, so that you get a consistent data. That’s how it should work.

Like in the following demo:

• SQL Fiddle Demo.

---

^{Note that: In all the demos in this answer, I used only the three tables involved with the trigger, also I modified the two fields inizio, fine to be nullable in the table TAPPA, because your insert clauses into that table have NULL values to those columns.}