I have a trigger like this:
IF ( value > new.value ) THEN
...
END IF
It will be executed AFTER INSERT. I need to pass value (it is a number) while inserting because of using it into the condition of the trigger. Any idea how can I pass it?
Noted that, currently it is hardcoded into trigger like IF ( 5 > new.value ) THEN. And all I'm trying to do is passing that 5 dynamically while inserting.
No, there's no defined mechanism for passing an argument to a TRIGGER.
The only things I can think of is a user-defined variable (but I wouldn't really want to go there), or a SELECT statement to retrieve the value from a table.
We can set a user-defined variable in our session:
SET #value_for_after_insert_trigger = 5 ;
Then perform an INSERT
INSERT INTO mytable (...) VALUES (...)
when the AFTER INSERT trigger is fired (for each row), the trigger body can reference a user-defined variable. For example:
DECLARE ln_value INTEGER;
SET ln_value = #value_for_after_insert_trigger;
Then we can do
IF ( ln_value > NEW.value ) THEN
Note that we aren't guaranteed that some other statement in our session won't modify our user-defined variable. For example, a BEFORE INSERT trigger might execute a statement such as:
SET #value_for_after_insert_trigger = 42 ;
... overwriting the value that was previously stored. The AFTER INSERT trigger would read the currently assigned value, getting the 42, not the 5 we specified earlier. And we aren't guaranteed that the AFTER INSERT trigger won't perform some action that modifies the value.
SET #value_for_after_insert_trigger = #value_for_after_insert_trigger + 1;
affecting the trigger execution for subsequent rows.
I do not advocate this as a design. I would avoid using user-defined variables like this.
Another alternative would be to use a SELECT statement to query the value from a table.
Related
I have this trigger to update a single field in a journal table after each update, but I need to update ALL columns in related row in journal at once, not by naming each of them and separating by comma, how can I do that?
DELIMITER //
CREATE TRIGGER sfo_clone_update_subtotal_invoiced AFTER UPDATE ON sales_flat_order
FOR EACH ROW BEGIN
update sales_flat_order_journal set subtotal_invoiced=NEW.subtotal_invoiced where entity_id=new.entity_id;
END;
DELIMITER ;
UPDATE syntax allows you to set multiple columns, separated by commas.
update sales_flat_order_journal
set subtotal_invoiced = NEW.subtotal_invoiced,
other_column1 = NEW.other_column,
other_column2 = (/* constant expression, not based on NEW */),
other_column3 = (SELECT ...scalar expression from some other tables... LIMIT 1),
updated_at = NOW()
where entity_id = new.entity_id;
You can access NEW.other_column to get the values of the same row that spawned the trigger.
You can DECLARE local variables in your trigger body to help calculate complex values.
You can use SELECT statements in your trigger body to query values from other rows or other tables, as long as you use a scalar SELECT that returns one row and one column.
If you need a more complex update that's too difficult to do in a trigger, I would do that in application code, not a trigger.
I was trying to create trigger which can update value of column user_count of table user_details using value of u_count of table user_info.
CREATE TRIGGER `test`
AFTER INSERT ON `user_details` FOR EACH ROW
BEGIN
DECLARE default_user_count int(11);
SELECT u_count INTO #default_user_count FROM
user_info WHERE user_info.id= user_details.id_c;
IF user_details.user_count= 0
THEN UPDATE user_details SET
user_count = default_user_count
WHERE user_details.id_c = user_info.id;
END IF;
END
Trigger saved successfully but when i tried to insert value in both table it is preventing to insert record into user_details means no row inserted in 2 this table, if we delete trigger then its working.
Can anyone let me know wrong with this trigger?
THanks,
M.
It's not really clear what you're trying to accomplish, but it seems like it's something like what we have below.
There are numerous errors in and ambiguities in your trigger.
Confusion on variables -- DECLARE default_user_count INT(11); does not declare the user-defined variable #default_user_count. It declares the program variable default_user_count. The # prefix references an entirely different variable scope and namespace.
SELECT and UPDATE from the table which invoked the trigger doesn't usually make sense (SELECT) or is completely invalid (UPDATE).
With in a trigger, you are operating FOR EACH ROW -- that is, for each row included in the statement that invoked the trigger. Inside an INSERT trigger, the NEW values for the row are in a pseudo-table/pseudo-row accessible via the alias NEW. For UPDATE triggers, there are NEW and OLD row values, and for DELETE triggers, just OLD.
AFTER INSERT doesn't seem to make sense. I think you're looking for BEFORE INSERT -- that is, while processing an INSERT INTO ... query, before the newly-inserted row actually gets written into the table, modify its values accordingly. The resulting row contains the original values except where the trigger has modified them.
SELECT ... INTO a variable is a practice you should not get into the habit of, because it can bite you in a way a scalar subquery can't, by leaving a variable unexpectedly unaltered instead of setting it to NULL as would be expected. In this case, it would have made no difference, but it's still a caution worth mentioning... and in this case, I've eliminated that intermediate variable altogether, so the subquery is the only option.
If you are trying to set a value in this table using a value found in another table, all you need to do is SET NEW.column_name equal to the value you want used in the row instead of the value provided with the insert statement.
CREATE TRIGGER `test`
BEFORE INSERT ON `user_details` FOR EACH ROW
BEGIN
IF NEW.user_count = 0 /* maybe also >> */ OR NEW.user_count IS NULL /* << this */ THEN
SET NEW.user_count = (SELECT ui.u_count
FROM user_info ui
WHERE ui.id = NEW.id_c);
END IF;
END
Again, it's unclear how the two tables are connected based on the content of the original question, but this appears to do what you're trying to accomplish.
There is a declared MySQL function GETUSERID() returning an integer value. How to make a record insert faster: setting the value from inside a query like
INSERT INTO ttable
(idtoset, some_other_field...)
VALUES (GETUSERID(), value1...);
or call
INSERT INTO ttable
(some_other_field...)
VALUES (value1...);
and fill idtoset by a trigger that fires before insert?
What if the query is performing multiple row insert like
INSERT INTO ttable
(idtoset, some_other_field...)
VALUES (GETUSERID(), value1...),
(GETUSERID(), value2...),
...
(GETUSERID(), valueN...);
?
Edit
I have just investigated the answer of #Rahul.
I created a ttest table with two triggers
CREATE TRIGGER `tgbi` BEFORE INSERT ON `ttest` FOR EACH ROW BEGIN
SET NEW.testint=1;
END;
CREATE TRIGGER `tgbi` BEFORE UPDATE ON `ttest` FOR EACH ROW BEGIN
SET NEW.testint=2;
END;
If I am not mistaken, should the before insert trigger call UPDATE SET the second trigger is expected to fire as well and the created testint value might be =2, but it is =1 in every inserted row. Could that mean that the engine optimises INSERT procedure and sets the value simultaneously with that set manually by query?
Appended on request of #Rick-James. The question is not about the definite function. It is actually about any function. Any function will be called same number of times if the record is inserted from trigger or from INSERT query. That is why I am wondering what is better from the point of MySQL engine - to call it manually setting the value in inserted records or filling it by means of triggers?
CREATE DEFINER=`***`#`***` FUNCTION `GETUSERID`() RETURNS int(10)
BEGIN
DECLARE id_no INT DEFAULT -1;
SELECT `id` INTO id_no FROM `tstuff`
WHERE `tstuff`.`user_name`=
(SELECT SUBSTRING_INDEX(USER(), '#', 1)) LIMIT 1;
RETURN id_no;
END
What is faster? No idea since I haven't done a bench marking on that but doing an direct INSERT operation would better to my knowledge instead of inserting and then perform an UPDATE through trigger.
Does what you are doing currently not working? you can as well make it a INSERT .. SELECT operation like
INSERT INTO ttable (idtoset, some_other_field...)
SELECT GETUSERID(), value1..., valuen FROM DUAL;
In past versions of MySQL, using a before insert trigger to populate a not nullable column didn't work as MySQL was evaluating the provided columns before the trigger. That's why whenever I have such a situation, I usually tend to go with functions instead of triggers.
From a performance point of view, since the before insert trigger is evaluated before actually writing data so the time needed to perform this is almost the same as immediately getting the value with the function and without trigger. But if all you are doing in the trigger is set the user ID, then I really see no reason to use a trigger.
I'm experiencing a race condition because I'm dealing with a lot of concurrency.
I'm trying to combine these two mysql statements to execute at the same time.
I need to select a row and update the same one...
SELECT id_file FROM filenames WHERE pending=1 LIMIT 1;
UPDATE filenames SET pending=2 WHERE id_file=**id of select query**;
Another solution to the race-condition I'm experiencing would be to perform an UPDATE query where pending=1 and somehow get the ID of the updated row, but I'm not sure if that's even possible?
Thanks
To deal with concurrency is one of the basic functions of transactions.
Wrap your queries into one transaction and tell the DBMS, that you need the row not to change in between with FOR UPDATE:
BEGIN;
SELECT id_file FROM filenames WHERE pending=1 LIMIT 1 FOR UPDATE;
# do whatever you like
UPDATE filenames SET pending=2 WHERE id_file=**id of select query**;
COMMIT;
You can execute these statements with 4 mysqli_query calls, and do whatever you want in between, without need to worry about the consistency of your database. The selected row is save until you release it.
You can avoid the "race" condition by performing just an UPDATE statement on the table, allow that to identify the row to modified, and then subsequently retrieve values of columns from the row.
There's a "trick" returning values of columns, in your case, the value of the id_file column from the row that was just updated. You can use either the LAST_INSERT_ID() function (only if the column is integer type), or a MySQL user-defined variable.
If the value of the column you want to retrieve is integer, you can use LAST_INSERT_ID() function (which supports a BIGINT-64 value).
For example:
UPDATE filenames
SET pending = 2
, id_file = LAST_INSERT_ID(id_file)
WHERE pending = 1
LIMIT 1;
Following the successful execution of the UPDATE statement, you'll want to verify that at least one row was affected. (If any rows satisfied the WHERE, and the statement succeeded, we know that one row will be affected. Then you can retrieve that value, in the same session:
SELECT LAST_INSERT_ID();
to retrieve the value of id_file column of the last row processed by the UPDATE statement. Note that if the UPDATE processes multiple rows, only the value of last row that was processed by the UPDATE will be available. (But that won't be an issue for you, since there's a LIMIT 1 clause.)
Again, you'll want to ensure that a row was actually updated, before you rely on the value returned by the LAST_INSERT_ID() function.
For non-integer columns, you can use a MySQL user-defined variable in a similar way, assigning the value of the column to a user-defined variable, and then immediately retrieve the value stored in the user-defined variable.
-- initialize user-defined variable, to "clear" any previous value
SELECT #id_file := NULL;
-- save value of id_file column into user-defined variable
UPDATE filenames
SET pending = 2
, id_file = (SELECT #id_file := id_file)
WHERE pending = 1
LIMIT 1;
-- retrieve value stored in user-defined variable
SELECT #id_file;
Note that the value of this variable is maintained within the session. If the UPDATE statement doesn't find any rows that satisfy the predicate (WHERE clause), the value of the user-defined variable will be unaffected... so, to make sure you don't inadvertently get an "old" value, you may want to first initialize that variable with a NULL.
Note that it's important that a subsequently fired trigger doesn't modify the value of that user defined variable. (The user-defined variable is "in scope" in the current session.)
It's also possible to do the assignment to the user-defined variable within in a trigger, but I'm not going to demonstrate that, and I would not recommend you do it in a trigger.
I have been trying to create a Trigger, however my attempts have been unsuccessful. I seem to be getting an error (#1064), which I have no solution for. Can somebody explain or demonstrate any faults in the syntax.
Let me specify:
I have delivery_id as primary key in delivery table,
I also have delivery_id as a foreign key in entry_log table.
By comparing both id's(if true), will return a text referring to the output of the bit (either 0 or 1)
DELIMITER //
DROP TRIGGER IF EXISTS entry_trigger//
CREATE TRIGGER entry_trigger BEFORE INSERT ON entry_log
FOR EACH ROW
BEGIN
DECLARE #xentry VARCHAR(45)
DECLARE #inta bit
SET #inta = SELECT allowed
FROM delivery
WHERE delivery.delivery_id = entry_log.delivery_id;
CASE
when #inta = 0 then #xentry = 'Acces Denied'
when #inta = 1 then #xentry = 'Acces Allowed'
END CASE
INSERT INTO entry_log(entry_time,access_allowed) VALUES(now(),#xentry);
END
//
This is assuming that you use MySQL. In the body of the trigger you use
WHERE delivery.delivery_id = entry_log.delivery_id;
I think you want to compare to the entry_log entry that the trigger is running on, right? In that case you must use this syntax:
WHERE delivery.delivery_id = NEW.delivery_id;
see here for more examples.
UPDATE
I see that also you try to do an INSERT INTO entry_log within the TRIGGER. This will of course not work, because you would create an infinite recursive loop. Within the
body of the trigger you can do unrelated table access, but not into the table you are inserting. You can change the values to be inserted by the trigger by setting NEW.xyz = whatever
UPDATE 2
I doubt, that your CASE statement is correct. At least it must end with END CASE. You can use IF here, since you don't have many cases to address. If you must use CASE this post might help you: MYSQL Trigger set datetime value using case statement
UPDATE 3
I am not sure, but I think you need brackets around the variable setting statement. try this trigger definition:
DELIMITER //
DROP TRIGGER IF EXISTS entry_trigger//
CREATE TRIGGER entry_trigger BEFORE INSERT ON entry_log
FOR EACH ROW
BEGIN
SET #inta = (SELECT allowed
FROM delivery
WHERE delivery.delivery_id = NEW.delivery_id);
SET NEW.access_allowed = #inta;
SET NEW.entry_time = NOW();
END
//
Note, that this is written out of my head, so beware of syntax errors in my script.