I have a mysql table:
`id` - int (primary key)
`datecolumn` - datetime
`name` - varchar
In my application, I want to enforce a rule that the name column can't be updated if the current date and time are past the datecolumn field's value. Currently, I'm querying the database for the value for each row, and then updating if the current date/time is before.
I'd like to know how to enforce this without the extra call to the database before updating. I'm updating many different rows at a time in the application, and think I'm causing performance issues because of all of the extra queries.
You can enforce rules like this using a trigger.
In your case, you would want a before update trigger. Something like this:
delimiter $$
create trigger noupdates before update on t
for each row
begin
if new.datecolumn > now() then
signal sqlstate '45000'
set message_text := 'Too late to update row';
end if;
end;
$$
delimiter ;
Related
I have a problem creating a trigger for a basic table that will check on insert if one of the values inserted is bigger than 3000 and replace it with 0. It throws this error:
Can't update table 'staff' in stored function/trigger because it is already used by statement which invoked this stored function/trigger
The structure of the table is very simple:
CREATE TABLE `staff` (
`ID` int(11) NOT NULL,
`NAZWISKO` varchar(50) DEFAULT NULL,
`PLACA` float DEFAULT NULL
)
And the trigger for it looks like this:
BEGIN
IF new.placa >= 3000 THEN
UPDATE staff SET new.placa = 0;
END IF;
END
I don't understand fully what occurs here, but I suspect some recursion, but I am quite new to the topic of triggers and I have lab coming, so I want to be prepared for it.
MySQL disallows triggers from doing UPDATE/INSERT/DELETE against the same table for which the trigger executed, because there is too great a chance of causing an infinite loop. That is, in UPDATE trigger, if you could UPDATE the same table, that would cause the UPDATE trigger to execute, which would UPDATE the same table, and so on and so on.
But I guess you only want to change the value of placa on the same row being handled by the trigger. If so, just SET it:
BEGIN
IF new.placa >= 3000 THEN
SET new.placa = 0;
END IF;
END
Remember that you must use a BEFORE trigger when changing column values.
I want to add a limit to how many items my table can have. I want the maximum amount of items to be 10. I want it to only be 10 people in my table. I dont want it to be able to add items after the 10th person. Here is my code:
CREATE TABLE person (
name VARCHAR(233) NOT NULL,
number int NOT NULL AUTO_INCREMENT,
PRIMARY KEY(number),
Check(number>10))
delimiter //
create trigger limit_persons before insert on person for each row
begin
declare count integer
select COUNT(*) FROM person INTO count;
if count>=10 then
signal sqlstate '45000' set message_text = 'Limit exceeded';
end if;
end
//
I would advise to handle such stuff as limitations in the software itself. So you have control over it later and it is overall a cleaner solution. But you can try this, if you really want to limit it in mysql:
ALTER TABLE tbl_name MAX_ROWS=10 AVG_ROW_LENGTH=nnn;
You can also check out triggers and signals:
https://dev.mysql.com/doc/refman/5.5/en/signal.html
https://dev.mysql.com/doc/refman/5.5/en/trigger-syntax.html
You can set up a trigger (to be specific, an Insert trigger) that counts the records and, if count is more than 10, it does not allow the insert operation.
Following code will be helpful to you,
DELIMITER $$
CREATE TRIGGER LimitRowCountTrigger
BEFORE INSERT
ON person
FOR EACH ROW
BEGIN
SELECT COUNT(*) INTO #cnt FROM person;
IF #cnt > 10 THEN
CALL sth(); -- raise an error
END IF;
END
$$
DELIMITER ;
I have this reservation table that has RESERVATION_ID , ROOM_NUM, Date_Start , Date_End and cost columns. what I want to do is insert all the columns except cost and fill the cost automatically.
delimiter //
CREATE TRIGGER upd_check before INSERT ON reservation
FOR EACH ROW
BEGIN
IF NEW.RESERVATION_COST = NULL THEN
SET NEW.RESERVATION_COST = 'timestampdiff(day, NEW.RESERVATION_STARTD, NEW.RESERVATION_ENDD)*70';
END IF;
END;//
delimiter ;
I wrote this trigger to do it, but whenever I press Apply to insert everything nothing is inserted to the RESERVATION_COST column.
why?
I would put this in a comment if I had enough reputation, but anyways. Triggers cannot act on the same table which activated them. Seems limiting, but we've had the same issue.
This link doesn't explicitly say this but it does say "in the associated table". Not the same: https://dev.mysql.com/doc/refman/5.0/en/triggers.html
You can't compare a value to NULL, you need to check if it IS NULL instead.
e.g.
IF NEW.RESERVATION_COST IS NULL THEN
I'd like to make a TIMESTAMP field DEFAULT CURRENT_TIMESTAMP, for 'creation time' purpose. But if someone or somehow something changes that TIMESTAMP, my data won't be consistent.
Is there a way I can ensure it won't change unless I delete the row and reinsert it, other than application level?
With the suggested answer provided, i could work around with something like this
CREATE TRIGGER consistency1 BEFORE UPDATE ON table1
FOR EACH ROW
BEGIN
IF NEW.creationtime != OLD.creationtime THEN
SET NEW.creationtime = OLD.creationtime;
END IF;
END;
Since my comment has been appreciated, here's the extended version.
I personally don't think that it's possible.
Anyway, there are a couple of things you can try:
Make sure that only your application can write on the database
Write a trigger like this (pseudocode!)
create trigger prevent_change_timestamp on tbl_name
before update
#fetch old row value
#verify if the timestamp field has been changed
#raise an error (any SQL error will do)
Or like this
create trigger revert_change_timestamp on tbl_name
after update
#fetch pre-change row value
#update the row with the "old" value in place of the new one
I'd personally go with the 3rd option, if possible. Anyway, the 2nd one is good too. I'd not rely on the 1st option unless necessary (eg: no access to trigger functionality)
More info here: reference
It's funny in a way that database apps don't offer this functionality as standard: not only for a "created" timestamp field, but for things like autoincrement id fields, and any miscellaneous values which you may want to set on creating a record and then never allow to be changed... wonder what the rationale is?
What you can do here is, you can write a TRIGGER on the table when a row is being updated. In that trigger, you can compare the old and new values, and if they are different then you can just overwrite the new value with the old one.
I tried this in MySQL 5.1 and got an error
DELIMITER //
CREATE TRIGGER member_update_0
-> AFTER UPDATE ON members
-> FOR EACH ROW
-> BEGIN
-> IF NEW.id != OLD.id THEN
-> SET NEW.id = OLD.id;
-> END IF;
-> END;//
ERROR 1362 (HY000): Updating of NEW row is not allowed in after trigger
The same trigger with AFTER replaced by BEFORE is accepted;
to me, this is a counter-intuitive way to do it, but it works
delimiter ;
UPDATE members SET id=11353 WHERE id=1353;
Query OK, 0 rows affected (0.00 sec)
Rows matched: 1 Changed: 0 Warnings: 0
It is actually possible to do this very neatly if you are using InnoDB.
Create another table with just one column. That column should have a foreign key (hence the innodb requirement in this solution) that points to the immutable column of the original table in question.
Put a restriction like "ON UPDATE RESTRICT".
In summary:
CREATE TABLE original (
....
immutable_column ...
INDEX index1(immutable_column)
....
) ENGINE=INNODB;
CREATE TABLE restricter (
.....
col1,
INDEX index2(col1),
FOREIGN KEY (col1) REFERENCES original (immutable_colum) ON UPDATE RESTRICT ON DELETE CASCADE
) ENGINE=INNODB;
Taking the idea a step further (for those of us still stuck with a legacy version of MySQL) you can have BOTH a protected & defaulted create_stamp AND an auto-updating update_stamp as follows:
If you have a table such as
CREATE TABLE `csv_status` (
`id` int(11) NOT NULL primary key AUTO_INCREMENT,
`create_stamp` datetime not null,
`update_stamp` timestamp default current_timestamp on update current_timestamp,
`status` enum('happy','sad') not null default 'happy'
);
Then you can define these triggers on it
drop trigger if exists set_create_stamp ;
create definer = CURRENT_USER trigger set_create_stamp BEFORE INSERT on
csv_status for each row
set NEW.create_stamp = now();
drop trigger if exists protect_create_stamp ;
delimiter //
create definer = CURRENT_USER trigger protect_create_stamp BEFORE UPDATE on
csv_status for each row
begin
if NEW.create_stamp != OLD.create_stamp then
set NEW.create_stamp = OLD.create_stamp;
end if;
end;//
delimiter ;
I have the following two tables in a MySql database:
Bookings
BookingID | ClientID | SeatID
SeatAvailability
SeatAvailabilityID | BookingID | ShowID | Available
They are linked on SeatID/SeatAvailabilityID.
I'm trying to write a trigger which updates the SeatAvailability table each time a row is inserted in Bookings. The trigger should change SeatAvailability.Available to 0 and also enter the BookingID from Bookings into the BookingID field in SeatAvailability with the same SeatAvailabilityID.
I've written this trigger, MySql accepts it but gives an error when inserting
"ERROR 1054: Unknown column 'cinemax.bookings.SeatID' in 'where clause'".
DELIMITER $$
USE `cinemax`$$
CREATE
DEFINER=`root`#`localhost`
TRIGGER `cinemax`.`update_available`
AFTER INSERT ON `cinemax`.`bookings`
FOR EACH ROW
UPDATE cinemax.seatavailability
SET cinemax.seatavailability.Availabe=0, cinemax.seatavailability.BookingID=cinemax.bookings.BookingID
WHERE cinemax.bookings.SeatID=cinemax.seatavailability.SeatAvailabilityID$$
try
AFTER INSERT ON `cinemax`.`bookings`
instead of
AFTER UPDATE ON `cinemax`.`bookings`
It's a couple of months late, but I decided to give it a quick shot before handing in the overall assignment. In the meantime I switched to postgres as it seemed to offer more functionality (albeit not as user friendly). I first had to create a trigger function:
CREATE OR REPLACE FUNCTION updateseatavailable()
RETURNS trigger AS
$BODY$
BEGIN
IF (TG_OP = 'INSERT') THEN
UPDATE "SeatAvailability"
SET "Available"='FALSE' AND "BookingID"=NEW."BookingID" WHERE "SeatAvailabilityID"=NEW."SeatID";
ELSIF (TG_OP = 'DELETE') THEN
UPDATE "SeatAvailability"
SET "Available"='TRUE' WHERE "SeatAvailabilityID"=OLD."SeatID";
END IF;
RETURN NEW;
END;
$BODY$
LANGUAGE plpgsql VOLATILE
and then simply call the function/procedure from a trigger:
CREATE TRIGGER UpdateSeatAvailable
AFTER INSERT OR DELETE ON "Bookings"
FOR EACH ROW
EXECUTE PROCEDURE updateSeatAvailable();
I wasn't able to get the BookingID in SeatAvailability to update for some reason (on Insert nothing happened and on Delete I got an error telling me Available cannot be null, even though I was changing the BookingID) so I omitted that in postgres,and implemented it with Java instead. It's not the best way but still better than nothing.
I decided to post my solution just in case someone has a similar problem and stumbles upon this question.