I'm currently working on an sqlite table where I have to do the following:
ID | Name | SortHint
---|-------|---------
0 | A | 1
1 | B | 2
2 | C | 3
ID is the primary key and SortHint is a column with the UNIQUE-constaint. What I have to do is to modify the table, for example:
ID | Name | SortHint
---|-------|---------
0 | A | 3
1 | B | 1
2 | C | 2
The Problem: Because of the UNIQUE I can't simply update one row after another. I tried:
BEGIN TRANSACTION;
UPDATE MyTable SET SortHint = 3 WHERE ID= 0;
...
COMMIT;
But the first update query immideatly fails with:
UNIQUE constraint failed: MyTable.SortHint Unable to fetch row
So, is there a way to "disable" the unique constaint for a transaction and only check all of them once the transaction is committed?
Notes:
I can't modify the table
It works if I only use SortHint values that are not already in the table
I know how to "workaround" this problem, but I would like to know if there is a way to do this as described above
One possibility is to drop the unique constraint and then add it again. That is a little bit expensive, though.
Another would be to set the values to negative values:
UPDATE MyTable
SET SortHInt = - SortHint;
UPDATE MyTable
SET SortHint = 3
WHERE ID = 0;
. . .
If you cannot modify the table, you are not able to remove the constraint.
A workaround could be to change the SortHint to a range that is not in use.
For example you could add 10,000 to all of them. Commit.
Then change to the right number at once which have become free now.
Maybe test afterwards that no numbers of 10,000 or higher exist anymore.
Related
I've created a table called Youtuber, the code is below:
create table Channel (
codChannel int primary key,
name varchar(50) not null,
age float not null,
subscribers int not null,
views int not null
)
In this table, there are 2 channels:
|codChannel | name | age | subscribers | views |
| 1 | PewDiePie | 28 | 58506205 | 16654168214 |
| 2 | Grandtour Games | 15 | 429 | 29463 |
So, I want to edit the age of "Grandtour Games" to "18". How can I do that with update?
Is my code right?
update age from Grandtour Games where age='18'
No, in update, you'll have to follow this sequence:
update tableName set columnWanted = 'newValue' where columnName = 'elementName'
In your code, put this:
update Channel set age=18 where name='Grandtour Games'
Comments below:
/* Channel is the name of the table you'll update
set is to assign a new value to the age, in your case
where name='Grandtour Games' is referencing that the name of the Channel you want to update, is Grandtour Games */
alter table changes the the schema (adding, updating, or removing columns or keys, that kind of thing).
Update table changes the data in the table without changing the schema.
So the two are really quite different.
Here is your answer -
-> ALTER is a DDL (Data Definition Language) statement
UPDATE is a DML (Data Manipulation Language) statement.
->ALTER is used to update the structure of the table (add/remove field/index etc).
Whereas UPDATE is used to update data.
Hope this helps!
I need help on a problem in my table definitions : I have a table which will be defined, for example, like this :
id, primary key
friend0_id, foreign key on table users
friend1_id, foreign key on table users
The problem is I do not want to have multiple times the couple (friend0_id, friend1_id), whatever the order the are in the table.
I tried to define an UNIQUE constraint on the couple (friend0_id, friend1_id), but the columns order defined in the constraint (here friend0_id, THEN friend1_id) matters. So :
| id | friend0_id | friend1_id |
|----|------------|------------|
| 1 | 3 | 4 | -> OK
| 2 | 4 | 3 | -> OK, as the columns order in index matters
| 3 | 3 | 4 | -> Not OK, constraint prevent this
I would like the id 2 and 3 in the example to be disallowed, but I can't figure how. Do you have some tips for me ?
Thank you,
naccyde
As #mu too short mentioned, the way is to use (greatest(friend0_id, friend1_id), least(friend0_id, friend1_id)), so, now I have a working 2 columns order free unique constraint. I did it in SQLite this way (which could not be the better) :
Create a trigger which set min(friend0_id, friend1_id) to friend0 and max(friend0_id, friend1_id) to friend1 :
CREATE TRIGGER friend_fixed_id_order_trigger
AFTER INSERT ON friends
BEGIN UPDATE friends
SET
friend0_id = min(NEW.friend0_id, NEW.friend1_id),
friend1_id = max(NEW.friend0_id, NEW.friend1_id)
WHERE friends.id = NEW.id;
END
Then, set a unique constraint on the couple (friend0_id, friend1_id) :
CREATE UNIQUE INDEX `friends_unique_relation_index`
ON `contacts` (`friend0_id` ,`friend1_id`)
And it works !
EDIT : If someone need this tip, do not forget to create an update trigger too, otherwise an update request could break the mechanism.
I have 3 tables in my database:
| systems |
|-------------|
| system_id |
|-------------|
| systems_maintenances |
|-------------------------|
| systems_maintenances_id|
| system_id |
| maintenance_id |
|-------------------------|
| maintenances |
|-------------------------|
| maintenance_id |
|-------------------------|
I'd like that when I delete a system, the maintenance being deleted too.
For the moment it only deletes in the systems_maintenances table.
I think a trigger is the best (and only?) way to do that.
I created this one:
CREATE TRIGGER `deleteMaintenance` BEFORE DELETE ON `systems`
FOR EACH ROW BEGIN
DELETE FROM maintenances
WHERE maintenance_id = systems_maintenances.maintenance_id;
END
The problem is I get the error:
DELETE FROM SNMProject.systems WHERE systems.system_id =16
MySQL a répondu:
#1054 - Unknown column 'systems_maintenances.maintenance_id' in 'where clause
How to solve my problem?
I don't know if it can help but I have this too for the systems_maintenances table:
(source: xooimage.com)
In a trigger, you don't refer to the table name of the table being affected. Instead, you refer to a proxie for it. That proxie depends on the database. In MySQL, it is new and old.
I suspect that you want something like this:
CREATE TRIGGER `deleteMaintenance` BEFORE DELETE ON `systems` FOR EACH ROW BEGIN
DELETE FROM maintenances m
WHERE maintenance_id IN (SELECT maintenance_id
FROM systems_maintenances sm
WHERE systems_maintenances.system_id = old.system_id
)
END;
There are other ways to express this logic, but this gives a good idea of the structure.
You can do essentially the same thing using cascading deletes, if you prefer. That way, you don't have to write triggers and can put the logic in the create table statement.
Is there any way to setup MySQL to every time a row is changed, then a row to another table/database is created with what the data was originally? (with time stamping)
If so how would I go about doing it?
E.g.
UPDATE `live_db`.`people`
SET `live_db`.`people`.`name` = 'bob'
WHERE `id` = 1;
Causes this to happen before the update:
INSERT INTO `changes_db`.`people`
SELECT *
FROM `live_db`.`people`
WHERE `live_db`.`people`.`id` = 1;
And if you did it again it would result in something like this:
`live_db`.`people`
+----+-------+---------------------+
| id | name | created |
+----+-------+---------------------+
| 1 | jones | 10:32:20 12/06/2010 |
+----+-------+---------------------+
`changes_db`.`people`
+----+-------+---------------------+
| id | name | updated |
+----+-------+---------------------+
| 1 | billy | 12:11:25 13/06/2010 |
| 1 | bob | 03:01:54 14/06/2010 |
+----+-------+---------------------+
The live DB needs to have a created time stamp on the rows, and the changes DB needs to have a time stamp of when the live DB row was updated.
The changes DB will also have no primary keys and foreign key constraints.
I'm using InnoDB and MySQL 5.1.49 but can upgrade if required.
Use a Trigger
MySQL support for triggers started with MySQL version 5.0.2.
You can create a trigger:
DELIMITER \\
CREATE TRIGGER logtrigger BEFORE UPDATE ON live_db.people
FOR EACH ROW BEGIN
INSERT INTO changes_db.people(id,name,updated) VALUES(OLD.id,OLD.name,now());
END;
\\
This is how I ended up doing it
DELIMITER |
# Create the log table
CREATE TABLE IF NOT EXISTS `DB_LOG`.`TABLE`
LIKE `DB`.`TABLE`|
# Remove any auto increment
ALTER TABLE `DB_LOG`.`TABLE` CHANGE `DB_LOG`.`TABLE`.`PK` `DB_LOG`.`TABLE`.`PK` INT UNSIGNED NOT NULL|
# Drop the primary keys
ALTER TABLE `DB_LOG`.`TABLE` DROP PRIMARY KEY|
#Create the trigger
DROP TRIGGER IF EXISTS `DB`.`update_TABLE`|
CREATE TRIGGER `DB`.`update_TABLE` BEFORE UPDATE ON `DB`.`TABLE` FOR EACH ROW
BEGIN
INSERT INTO `DB_LOG`.`TABLE`
SELECT `DB`.`TABLE`.*
FROM `DB`.`TABLE`
WHERE `DB`.`TABLE`.`PK` = NEW.`PK`;
END|
DELIMITER ;
Sorry to comment on an old post, but I was looking to solve this exact problem! Thought I would share this information.
This outlines a solution perfectly:
http://www.hirmet.com/mysql-versioning-records-of-tables-using-triggers
Database is MS SQLServer
Data example:
| Name | defaultValue | value |
| one | true | valone |
| one | false | valtwo |
| one | false | valthree |
I'm after a way of constraining the table such that each 'Name' can only have one row with 'defaultValue' set to true
Create a computed column like this:
ALTER TABLE yourtable
ADD ValueCheck AS CASE defaultValue
WHEN true THEN 1
WHEN false THEN NULL
END
and then add unique constraint for (Name, ValueCheck)
I liked Michael's idea but it will only allow you one false value per name in SQL Server. To avoid this how about using
ALTER TABLE yourtable
ADD [ValueCheck] AS
(case [defaultValue] when (1) then ('~Default#?#') /*Magic string!*/
else value end) persisted
and then add unique constraint for (Name, ValueCheck).
I am assuming that name, value combinations will be unique. If the value column does not allow NULLs then using NULL rather than the magic string would be preferable otherwise choose a string that cannot appear in the data (e.g. 101 characters long if the value column only allows 100 chars)
You can use a TRIGGER to validate this constraint on update or insert events and roll back the transaction if it was invalid.