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 ;
Related
I have a program in Laravel where after users register they need a badge number, I want that number to be generated randomly after they register in the database. I should use triggers but I struggle with syntax.
users table
id bigint(20)
name varchar(255)
surname varchar(255)
nr_legitimatie varchar(255)
I want that 'nr_legitimatie' field to be unique.
This is what I tried but with no success
Trigger
CREATE OR REPLACE TRIGGER numar_leg
AFTER INSERT ON users
FOR EACH ROW
DECLARE
legitimatie VARCHAR(191)
BEGIN
legitimatie =('
SELECT FLOOR(RAND() * 99999) AS random_num
FROM numbers_mst
WHERE "random_num" NOT IN (SELECT my_number FROM numbers_mst)
LIMIT 1' );
set `users`.`nr_legitimatie` = legitimatie;
END;
Here's an example of a MySQL BEFORE INSERT trigger that assigns a value to the nr_legitimatie column.
DELIMITER $$
DROP TRIGGER numar_leg$$
CREATE TRIGGER numar_leg
BEFORE INSERT ON users
FOR EACH ROW
BEGIN
DECLARE li_nrn BIGINT DEFAULT NULL;
DECLARE li_cnt BIGINT DEFAULT 1;
WHILE li_cnt > 0 DO
-- generate a new random number
SELECT FLOOR(RAND()*99999) AS nrn INTO li_nrn;
-- check if the new random number is already used
SELECT COUNT(1) INTO li_cnt FROM users u WHERE u.nr_legitimatie = li_nrn;
END WHILE;
SET NEW.nr_legitimatie := li_nrn;
END$$
DELIMITER ;
Note that this does not guarantee that the value assigned to the nr_legitimatie will be unique, because the code in the trigger is subject to a race condition. There is potential for two (or more) simultaneous sessions to each discover the same random number is not yet "unused", and each session will use it. (The check for an existing value precedes the assignment to the column.)
If we want to guarantee uniqueness, we should add a UNIQUE constraint (UNIQUE KEY) on the column in the users table.
We can also use a separate table to track the numbers that are used, with a UNIQUE constraint on the column, we can attempt inserts, and catch the error when an attempt to insert a duplicate is made.
If we introduce a tracking table, e.g.
CREATE TABLE nrn (nrn BIGINT PRIMARY KEY) ;
Then we can avoid the race condition, making the test for existing duplicate and reservation of the new value at the same time. Something like this:
DELIMITER $$
CREATE TRIGGER numar_leg
BEFORE INSERT ON users
FOR EACH ROW
BEGIN
DECLARE li_nrn BIGINT DEFAULT NULL;
DECLARE li_dup BIGINT DEFAULT 1;
DECLARE CONTINUE HANDLER FOR 1062 SET li_dup := 1;
WHILE li_dup > 0 DO
SELECT FLOOR(RAND()*99999) AS nrn INTO li_nrn;
SET li_dup := 0;
INSERT INTO nrn (nrn) VALUES (li_nrn);
END WHILE;
SET NEW.nr_legitimatie := li_nrn;
END$$
DELIMITER ;
The edge case here is the trigger is executed, a new random number is generated and reserved, but the insert into the users table fails for some reason, and we don't issue a ROLLBACK. If we issue a ROLLBACK, then our new random number reservation will also be rolled back (unless nrn is a MyISAM table).
I have a table warehouse where I have information about articles in my store (article id as foreign key and quantity). Then, I have another table, shoppinglist where I have a clients id, article id and quantity. Lets say, that client wants to buy 3 articles but theres only one article available. How to write a trigger which help me to prevent from buying too much?
I tried this:
DELIMITER $$ CREATE TRIGGER check BEFORE INSERT ON shoppinglist FOR EACH ROW BEGIN IF warehouse.quantity < shoppinglist.quantity THEN CALL fail('You cant buy that much'); END IF; END $$ DELIMITER;
but this seems not to work. I mean, when I do:
INSERT INTO shoppinlist (clients_id, article_id, quantity) VALUES (1, 2, 100);
having only 2 articles with id = 2 on warehouse its ok, its possible. What did I do wrong?
What specific article would warehouse.quantity or shoppingList.quantity refer to in your code?
Also, check is a reserved keyword.
Try this:
DELIMITER $$
CREATE TRIGGER qtyCheck BEFORE INSERT ON shoppinglist
FOR EACH ROW
BEGIN
SET #qty = (SELECT quantity FROM warehouse WHERE article_id = NEW.article_id);
IF #qty < NEW.quantity THEN
CALL fail('You cant buy that much');
END IF;
END $$
DELIMITER ;
Note that I renamed the trigger, I'm guessing the name of the article_id column on the warehouse table, I used the NEW variable instead of shoppingList within the body of the trigger, and you need a space before the semicolon in DELIMITER ;, though this might've been a typo when posting.
Finally, you may get the following error if the fail function isn't defined. It doesn't exist on my system...
ERROR 1305: PROCEDURE testing.fail does not exist
I am working on a project where I need my ID column to be a power of 2 (1,2,4,8,16..). I know that we cannot offset the auto_increment but for simple addition/subtraction in my.cnf.
Example:
id
----
1
2
4
8
16
32
64
128
etc
One of the ideas I had was to use the auto increment functionality as the base, and then create a trigger to apply the power of 2 and update the new ID, but unfortunately, it is not working:
DELIMITER $$
CREATE TRIGGER testbitcompatid BEFORE INSERT ON Table
FOR EACH ROW
BEGIN
SET NEW.id = pow(NEW.id, 2)
END;
$$
DELIMITER ;
Because the BEFORE INSERT has not yet generated the AUTO_INCREMENT id, the AUTO_INCREMENT will always return 0, essentially causing no change on the columns.
I also tried AFTER INSERT:
DELIMITER $$
CREATE TRIGGER testbitcompatid AFTER INSERT ON Table
FOR EACH ROW
BEGIN
SET Table.id = pow(NEW.id, 2) WHERE id = NEW.id;
END;
$$
DELIMITER ;
But this failed because you cannot change values of the table which the trigger is applied to during an AFTER INSERT.
Scratching my head, but I am sure someone else has a great way of accomplishing this.
To work around all the issues above, I was able to construct the following which works great!
DELIMITER $$
CREATE TRIGGER testbitcompatid BEFORE INSERT ON Table
FOR EACH ROW
BEGIN
SET #LAST_ROW = (SELECT MAX(id) FROM Table);
SET NEW.id = CASE WHEN #LAST_ROW IS NULL THEN 1 ELSE #LAST_ROW * 2 END;
END;
$$
DELIMITER ;
Pretty much, we take the highest id, grab the log(2) of it which gives us the corresponding AUTO_INCREMENT id. We then add 1, and power that up to 2.
I hope this helps prevent some headache down the road for others.
I created my ER and all my tables in MySQL but I need to know how to set a max number allowed for one of my attributes.
The attribute is a vehicle and I want my database to set a limit to 10 and not allow the user to add an 11th car.
I'm not too familiar with triggers. How can I start?
Why don't you just count the number of rows in your table before inserting?
If the count return 10, don't insert.
Mysql doesn't provide a mechanims to prevent a INSERT INTO a table with triggers.
A solution is use a trigger with a not null column.
CREATE TRIGGER carsCheck BEFORE INSERT ON cars
for each row
begin
DECLARE counter INT;
SET counter = (select count(id) from cars);
if counter > 10 then
NEW.notNulCol = null;
end if;
end;
I have an issue with a trigger on a mysql database. I have a table such as follows:
id int not null auto_increment (PK)
parent_id int not null,
rank int not null
What I'm trying to do is use a trigger to update the rank to the next highest +10 when they have the same parent_id, but this doesn't seem to be working.
DELIMITER $$
DROP TRIGGER IF EXISTS after_insert $$
create trigger after_insert
after insert on mytable
FOR EACH row
BEGIN
IF EXISTS (SELECT rank FROM mytable WHERE parent_id = new.parent_id AND id != new.id ORDER BY rank DESC LIMIT 1) THEN
UPDATE mytable SET rank = 10
WHERE id = new.id;
ELSE
UPDATE mytable SET rank = 20
WHERE id = new.id;
END IF;
END
$$
I've tried setting the new rank to a variable and calling the update statement using that, and again it didn't work. I even created another table to log what values were being selected and that worked perfectly so I can't quite understand what's going on. Is it a case of, although the trigger is "AFTER INSERT" the insert hasn't actually happened so it can't update the row it's just inserted? Another reason I ask this is, I've even tried updating the rank to different values e.g 1 and 2 depending on which statement it goes to, but it always ends up being 0.
I think you're on the right track with this thought:
Is it a case of, although the trigger is "AFTER INSERT" the insert hasn't actually happened so it can't update the row it's just inserted?
From the FAQ:
B.5.9: Can triggers access tables?
A trigger can access both old and new data in its own table. A trigger can also affect other tables, but it is not permitted to modify a table that is already being used (for reading or writing) by the statement that invoked the function or trigger.
The documentation isn't clear that what you're doing won't work. OTOH, the documentation isn't clear that what you're trying to do will work either.
I think you'd be better off using a BEFORE INSERT trigger and setting NEW.rank in there. Then, the new row would have the right rank value when it is actually inserted into the table rather than patching it after. Also, you'd be able to simplify your existence check to just this:
EXISTS(SELECT rank FROM mytable WHERE parent_id = new.parent_id)
as NEW.id wouldn't have a useful value and the new row wouldn't be in the table anyway; the ORDER BY and LIMIT are also unnecessary as you're just checking if something exists so I took them out.
A BEFORE INSERT trigger seems to match your intent better anyway and that will give you correct data as soon as it is inserted into your table.
If you want the rank to be set +10 more than highest "brother's" rank, you could use:
DELIMITER $$
DROP TRIGGER IF EXISTS whatever $$
create trigger whatever
BEFORE INSERT ON mytable
FOR EACH row
BEGIN
SET NEW.rank = 10 + COALESCE(
( SELECT max(rank)
FROM mytable
WHERE parent_id = NEW.parent_id
), 0 ) ;
END
$$