So, I need to safely restrict the insertion of entries in a table based on the count of other entries in that same table. Say we have the following table:
resource:(id, foreign_key)
I need to create up to a number of entries based on the foreign key. So, as soon as I reach a count, let's say 100 for our example, I want to restrict creating more entries.
The obvious answer would be something like that:
count the entries with the specified foreign key.
if count < limit insert the new entry
And in fact, that's what I have been using. The thing is, this approach is not fail-proof since between 1 and 2 there might occur another insertion. I considered the possibility of using transactions but (unless I'm completely misunderstanding transactions) this has the same issue:
start transaction
insert the new entry
if entries have exceeded the limit, rollback. otherwise commit
Now, say we already have 99/100 entries and two transactions run at the same time. They both will commit since they don't see each-other's entries.
Short of actually creating the entry and then delete it if it's invalid (which feels kindof messy in my mind) I can't think of a way to solve this issue. Any ideas?
edit: upon request I'm providing sample data:
table1
+-------------+------------------+------+-----+----------------+
| Field | Type | Null | Key | Extra |
+-------------+------------------+------+-----+----------------+
| id | int(10) unsigned | NO | PRI | auto_increment |
| limit | int(10) unsigned | NO | MUL | |
+-------------+------------------+------+-----+----------------+
table2
+-------------+------------------+------+-----+----------------+
| Field | Type | Null | Key | Extra |
+-------------+------------------+------+-----+----------------+
| id | int(10) unsigned | NO | PRI | auto_increment |
| foreign_id | int(10) unsigned | NO | MUL | |
+-------------+------------------+------+-----+----------------+
and some sample data:
table1
+----+----------+
| id | limit |
+----+----------+
| 1 | 5 |
+----+----------+
table2
+----+---------------+
| id | foreign_id |
+----+---------------+
| 1 | 1 |
+----+---------------+
| 2 | 1 |
+----+---------------+
| 3 | 1 |
+----+---------------+
| 4 | 1 |
+----+---------------+
At this point, let's say that two users attempt to create table2 entries. The first one will have to be accepted and the 2nd rejected.
With the first approach, if both users go through step 1 (counting the old entries) and then through step 2 (insert the new entry) both entries will be created.
With the second approach, if both of them run at the same time, they both will count 4 slots before themselves and commit instead of one of them rollbacking.
Halo Mate, a Stored Procedure similar to this structure may help you
UPDATE
DROP PROCEDURE IF EXISTS sp_insert_record;
DELIMITER //
CREATE PROCEDURE sp_insert_record(
IN insert_value1 INT(9),
IN chosen_id INT(9)
)
BEGIN
SELECT id, `limit`
INTO #id, #limit
FROM table1
WHERE id = chosen_id;
START TRANSACTION;
INSERT INTO table2 (id, foreign_id)
VALUES (insert_value1, chosen_id);
SELECT COUNT(id)
INTO #count
FROM table2
WHERE foreign_id = #id;
IF #count <= #limit THEN
COMMIT;
ELSE
ROLLBACK;
END IF;
END//
DELIMITER ;
By using a Stored Procedure, you can also add any validation or process based on your requirements.
Hope this can be of help, cheers!
Related
I have 2 tables which are below.
I would like to create a trigger for table_one. when changed the username in table_one, update user_id value from the table_two user value.
table_one:
+--------------+----------+
| user_id | username |
+--------------+----------+
| 15 | robin |
| 46 | albert |
+--------------+----------+
table_two:
+--------------+----------+
| id | user |
+--------------+----------+
| 1 | john |
| 2 | jack |
| 3 | robin |
| 4 | kylie |
| 5 | robert |
| 6 | albert |
| 7 | jay |
+--------------+----------+
thanks in advance
Do a BEFORE UPDATE trigger, not an AFTER UPDATE trigger.
(I don't think it's possible in an AFTER UPDATE trigger to modify the row that was updated, that fired the trigger. I could be wrong about that, but I just can't wrap my brain around how that would work.)
This is a demonstration of a BEFORE UPDATE trigger on table_one that assigns a value to the user_id column (of the row being updated) based on the result from a query:
DELIMITER $$
CREATE TRIGGER ...
BEFORE UPDATE ON table_one
BEGIN
# local variable we will temporarily store a value fetched from a query
DECLARE li_new_id BIGINT DEFAULT NULL; # match datatype of table_two.id
# lookup `id` value from `table_two` with a SQL query
SELECT s.id
INTO li_new_id
FROM table_two s
WHERE s.user = NEW.username
ORDER BY s.id
LIMIT 1 ;
# assign the value we fetched to the `user_id` column of the row being updated
SET NEW.user_id := li_new_id ;
END$$
DELIMITER ;
Note that if the query doesn't find a matching row, the local variable will have a NULL value. So the SET statement will assign NULL to to the user_id column. There's no check in the trigger that the value we assign won't violate a constraint (NOT NULL, UNIQUE, FOREIGN KEY).
I've been trying to implement a simple script that locks a table from being read, updates some fields, and then unlocks it.
Here's my table:
mysql> SHOW COLUMNS FROM tb1;
+---------------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------------+---------+------+-----+---------+-------+
| id | int(11) | YES | PRI | NULL | |
| status | int(1) | YES | | 0 | |
+---------------+---------+------+-----+---------+-------+
Lets see if you guys understand what I'm trying to do:
Start transaction
SELECT all rows with status != 1 (it may return more than 1 row) with FOR UPDATE statement;
UPDATE the field status of the rows that has been selected in pass 2
Commit
I tried to achieve this in many ways, but I cant persist the SELECT data that I got in pass 2 and I can't use SELECT ... FOR UPDATE as a subquery of a UPDATE like this UPDATE tb1 SET status=1 WHERE id IN (SELECT id FROM tb1 WHERE status != 1 FOR UPDATE);
Is it possible to achieve this instead of updating row by row?
I have a MySQL table contacts, with structure as follows
+--------------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+--------------+----------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| contactee_id | int(11) | NO | MUL | 0 | |
| contacter_id | int(11) | NO | MUL | 0 | |
+--------------+----------+------+-----+---------+----------------+
contactee_id and contacter_id are both ids, which together defines a relationship between two users. In order to calculate the count of relations, a user have, I have the following query
INSERT INTO followers (id, followers)
SELECT contactee_id, 1
FROM contacts
ON DUPLICATE KEY
UPDATE followers = followers + 1
The problem with this query is that it locks the contacts table for too long (more than 16 minutes). I want to get it done in batches, so that the SQL does not locks contacts table for too long. Few ways, I thought of, but they all need to lock the entire table. Is there a way this could be done?
If you just want the count of relations use the count and group by together like
SELECT contactee_id,count(contacter_id) FROM contacts group by contactee_id;
This will give you all the contactee_id and the number of contacter_id's for each contactee
Run query for some records and then save the id of the last record in a table or filesystem, start next query from that id and update it every cycle.
I want to be able to insert data into t1 and have data get populated in table t2 with the primary key as a foreign key in t2.
Basically, how come in my current setup when I INSERT INTO t1 (first_name, last_name) values ( "blah", "blah"); and then do SELECT * FROM t2; t2 it says Empty Set (0.00 sec) for t2? Shouldn't it at least show the default id of 1?
t1:
+------------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+------------------+------+-----+---------+----------------+
| first_name | varchar(20) | NO | | NULL | |
| last_name | varchar(20) | NO | | NULL | |
| id | int(10) unsigned | NO | PRI | NULL | auto_increment |
+------------+------------------+------+-----+---------+----------------+
t2:
+-----------+------------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+------------------+------+-----+---------+-------+
| address | varchar(50) | NO | | NULL | |
| id | int(10) unsigned | NO | MUL | NULL | |
| last_name | varchar(20) | YES | | NULL | |
+-----------+------------------+------+-----+---------+-------+
In a relational database, a FOREIGN KEY is a declaration that you intend to insert values into T2 that must match an already existing value in T1, and that you want the database to refuse to perform any action that would break this relationship.
It does not mean that the database will create records on its own in order to satisfy a relationship. If you try to insert a value into T2 that does not exist in T1, the command will fail; it will not add the required record to T1.
That is the opposite of what you're suggesting, however, in which you want the foreign key values to get automatically generated. However, there's no requirement that a primary key value actually have references and, furthermore, no limit on the number of times that primary key value can be referenced — so how would the database guess what should be created in T2?
That said, if you want some of your own code to execute automatically when data is added to T1, code which can do whatever you want, you can create a trigger on T1.
No, tables won't propagate automatically. (You can however do it with triggers) You will have to insert into t2.
You can create a trigger on table t1 so that it inserts a row into t2 with the correct id and the other fields NULL
Foreign keys will not insert records for you.
DELIMITER ;;
CREATE TRIGGER insert_addr_rec BEFORE INSERT ON t1
FOR EACH ROW BEGIN
INSERT INTO t2 SET id=NEW.id, last_name=NEW.last_name
END ;;
DELIMITER ;
NB untested code
There are the following two tables:
create table lol(id int auto_increment, data int, primary key id(id));
create table lol2(id int auto_increment, data int, primary key id(id));
Insert some values:
insert into lol2 (data) values (1),(2),(3),(4);
Now insert using select:
insert into lol (data) select data from lol2;
Do it again:
insert into lol (data) select data from lol2;
Now look at the table:
select * from lol;
I receive:
+----+------+
| id | data |
+----+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
| 8 | 1 |
| 9 | 2 |
| 10 | 3 |
| 11 | 4 |
+----+------+
I'm puzzled by the gap between 4 and 8... What caused this and how can I do it so that there isn't a gap? Thanks a lot!
auto_increment does not guarantee to have increments by 1 in the ID column. And it cannot, because as soon as you work with parallel transactions it would break anyways:
BEGIN BEGIN
INSERT INTO lol VALUES(...) INSERT INTO lol VALUES(..)
... ...
COMMIT ROLLBACK
What ids should be assigned by the database? It cannot know in advance which transaction will succeed and which will be rolled back.
If you need a sequential numbering of your records you would use a query which returns that; e.g.
SELECT COUNT(*) as position, lol.data FROM lol
INNER JOIN lol2 ON lol.id < lol2.id
GROUP BY lol.id