Mysql – Using count in triggers to update another table

countMySQLtrigger

Say i have a table called trx_request with the following data

id | confirmed(TIME) | finished(TIME)
-------------------------------------
1  | 12:05:00        | NULL
2  | 13:11:00        | NULL

and another table called trx_updates with the following data

request(FK) | role(INT) | added(TIME)
------------------------------------------
1           | 1         | 14:02:00

I'd like to make a trigger that after a record is inserted into trx_updates, counts how many records has the same value in the request field (such as how many '1's there are). And then, does a check to see if the count reaches 4 ('1' shows up 4 times). When it does, it updates the trx_requests.finished to NOW().

How would one make a count statement in triggers? or are triggers the way to go?

Best Answer

This trigger does the job:

create trigger trx_updates_atrig
after insert on trx_updates for each row begin
DECLARE updatecount INT;
  set updatecount = ( select count(*) from trx_updates where request = new.request );
  if updatecount=4
    then
      update trx_request set finished=NOW() where id = new.request;
  end if;
end//

SQLFiddle Demo.

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Mysql – Synchronise data from one table to another & vice-versa using triggers

I concur with @NathanJolly, this is a potential data nightmare and seems ill-advised... but there is a way to implement this that is, at least conceptually, fairly straightforward.

You need not worry about the possibility of recursion in triggers, because this won't happen... MySQL doesn't support it.

I built this fiddle that creates tables t1 and t2, each of which has an AFTER INSERT trigger that tries to insert the record into the other table... then tries to do an insert. Theoretically, there's a potential for recursion, but MySQL prevents this:

ERROR 1442 (HY000): Can't update table 't1' in stored function/trigger because it is already
                    used by statement which invoked this stored function/trigger.

So it won't run in a loop, but it still throws an exception. Technically, it's not trigger recursion causing the exception, because we haven't actually gotten that far... it's the fact that the trigger in t2 tries to modify table t1, which was involved in the currently-executing statement.

Then again, technically, it's not just the recursion of the triggers we need to avoid -- we have to stop one step before that -- if the client modifies t1, the t1 trigger modifies t2, then t2's trigger needs to not even try to do anything to t1 (including, but not limited to, firing the trigger again on t1).

Implementing this is actually fairly straightforward... in each trigger, we toggle the value of a session to tell the other trigger not to run.

CREATE TRIGGER t1_ai AFTER INSERT ON t1 FOR EACH ROW BEGIN
  IF @__disable_trigger_t1t2 = 1 THEN
    SET @__disable_trigger_t1t2 = NULL;
  ELSE
    SET @__disable_trigger_t1t2 = 1;
    -- trigger logic goes in here
    INSERT INTO t2 (id,things) VALUES (NEW.id,NEW.stuff);
  END IF;
END //

CREATE TRIGGER t2_ai AFTER INSERT ON t2 FOR EACH ROW BEGIN
  IF @__disable_trigger_t1t2 = 1 THEN
    SET @__disable_trigger_t1t2 = NULL;
  ELSE
    SET @__disable_trigger_t1t2 = 1;
    -- trigger logic goes in here
    INSERT INTO t1 (id,stuff) VALUES (NEW.id,NEW.things);
  END IF;
END //

A test case for this logic working can be seen in this fiddle.

Essentially, when a trigger fires, it checks the value of the @__disable_trigger_t1t2 variable (this variable name is, of course, arbitrary). If '1', it sets it back to NULL (the default for session variables) and does no further processing, because the '1' means this row is being manipulated by the trigger on the other table. If already NULL, then it sets the value to 1 and executes the trigger logic -- which will update the other table, whose trigger will see the '1', will not execute its own logic, and will reset the variable back to NULL, so that the next execution of either trigger -- whether for the next row in the same statement, or for a subsequent statement -- will see the NULL and execute normally.

In MySQL, triggers are only supported FOR EACH ROW, meaning that we can define a MySQL trigger as "a stored program that is executed before or after each row is inserted, updated, or deleted"... which means that no matter how many rows your query affects, the trigger will run once for each row. Actions against other tables will cause the triggers on those tables to fire, and the execution of the trigger on t1 for the first row will not complete until all of the triggers it caused to fire have also completed... and this is why we can use this variable like this -- only one row is actually being handled at a time, so we set and clear this variable for each row, always leaving it NULL after each statement terminates.

A significant caveat is that this approach fails if you need to trigger on cascading deletes or updates related to foreign keys, because:

"Currently, cascaded foreign key actions do not activate triggers."

^{-- http://dev.mysql.com/doc/refman/5.6/en/create-trigger.html}

So, while I have misgivings about the long-term viability of maintaining database synchronization with this approach because of the number of things that could go wrong with two applications manipulating each other's data or logical errors in the trigger definitions... it seems technically possible.

You also mentioned:

views cannot have triggers in MySQL

No, but updating an updatable view will fire the triggers on the underlying base table as if the table had been directly manipulated.

Mysql – using triggers to synchronize data

Well thanks everyone, but I read somewhere saying that i should include this in the table i want to sync to(to make the table unique) "alter table "YOURTABLE add unique index(row1, row2);"

then i add ON DUPLICATE KEY UPDATE name = name and number = number;

Here is a final trigger after making the table unique to the one am synchronize to, in this case it's "database2.cc_phonenumber" and everything worked perfectly.

    delimiter |

CREATE TRIGGER database1 BEFORE INSERT ON pesapi_payment
  FOR EACH ROW
  BEGIN
    UPDATE database2.cc_phonenumber SET amount=amount+new.amount where id=new.amount
    INSERT INTO database2.cc_phonenumber(number, name, amount) values (new.phonenumber, new.name, new.amount) ON DUPLICATE KEY UPDATE name = name and number = number;
       END;
    |


delimiter ;

Best Answer

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Mysql – Synchronise data from one table to another & vice-versa using triggers

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