i want to create triggers and i have a requirement to do the following.
When a record in a tables is edited trigger that will automatically log this change to another table called logs and where it use inside my php script
The set up of the log table will be as follows:
date
time
value before
value after
edited table
* i am new for triggers please help me my project
Your google-fu is weak my friend: http://dev.mysql.com/doc/refman/5.0/en/create-trigger.html but you will learn eventually :)
Though if this is not a project dedicated to creating triggers, I would not advice to use them. Since this " that will automatically log this change to another table called " can surely be done in business logic. Then you can control what is done from software and by the user. Maybe in the near future you will need to perform a datafix on your table - then you would have to disable the trigger, perform the fix and enable it again. That seem like a lot of work you don't really want to do.
So if possible - avoid triggers, if not - read the mysql docs on how to create them.
Related
I have several databases that are used by several applications (one of which is our own, the others we have no control over in what they do).
Out software has to know when the database has last been changed. For reasons I won't get into to keep this short we decided that going with a new table per database that has a singular field: last_changed_on that has a GetDate() as a value. This way our own software can check when it was last changed and check it to the date it has stored for said database and do things if the date is newer than what is stored in-memory.
After doing some research we decided that working with Triggers was the way to go, but from what I could find online, triggers look at specific columns that you set for Updates.
What I'd like to know is if there is a way to automate the process or just have a trigger that happens whenever anything happens insert, update, remove wise?
So I am looking for something like this:
CREATE TRIGGER LastModifiedTrigger
ON [dbo].[anytable]
AFTER INSERT, UPDATE, DELETE
AS
INSERT INTO dbo.LastModifiedTable (last_modified_on) VALUES (CURRENT_TIMESTAMP)
I know that the above example isn't a correct trigger, I'm rather new to them so I was unsure on how to word it.
It might be interesting to note that I can have my own software run several queries creating the queries automatically for each table and each column, but I'd rather avoid to do that as keeping track of all those triggers will be a pain in the long run.
I'd prefer to have a little triggers per database as possible, if only by not having to make a trigger for each individual column name.
Edit: To clarify: I am trying to avoid having to create an automated script that goes and scans every table, and sequentially every column of every table, to create a trigger to see if something is changed there. My biggest issue at the moment is the trigger behavior on updates, but I'm hoping to avoid having to specify tables as well for insert and delete
Edit 2: To avoid future confusion, I'm looking for a solution to this problem for both SQL Server (MS SQL/T SQL) and MySQL
Edit 3: Turns out that I read the documentation very wrongly and (at least on MySql) the trigger activates on any given updated column without having to define a specific one. Regardless, I'm still wondering if there is a way to just have less triggers than having one for each table in a database. (i.e. 1 for any type of update(), 1 for any type of insert(), and 1 for any type of delete()
EDIT 4: Forgot that the argument for overwriting 1 field will come with performance issues, I've considered this and I'm now working with multiple rows. I've also handled the creating of 3 triggers (insert(), update(), and delete()) for each database through my software's code, I really wished this could've been avoided, but it cannot.
Solution
After a bunch more digging on the internet and keep finding opposite results of what I was looking for, and a bunch of trial and error, I found a solution.
First and foremost: having triggers not being dependent on a table (aka, the trigger activates for every table is impossible, it cannot be done, which is too bad, it would've been nice to keep this out of the program code, but nothing I can do about it.
Second: the issue for updates on not being column specific was an error due to my part for searching for triggers not being dependent on specific columns only giving me examples for triggers that are.
The following solution works for MySql, I have yet to test this on SQL Server, but I expect it to not be too different.
CREATE TRIGGER [tablename]_last_modified_insert
AFTER INSERT/UPDATE/DELETE ON [db].[tablename]
FOR EACH ROW
BEGIN
INSERT INTO [db].last_modified(last_modified_on)
VALUES(current_timestamp())
END
As for dynamically creating these triggers, the following show how I get it to work:
First Query:
SHOW TABLES
I run the above query to get all the tables in the database, exclude the last_modified I made myself, and loop through all of them, creating 3 triggers for each.
A big thank you to Arvo and T2PS for their replies, their comments helped by pointing me in the right direction and writing up the solution.
You're slightly off in the assumption that SQL Server triggers are per-column; the CREATE TRIGGER syntax binds the trigger to the named table for the specified operations. The trigger will be called with two logical tables in scope (inserted & deleted) that contain the rows modified by the operation that caused the trigger to fire; if you wanted to check for specific columns' values or changes, then the trigger logic would need to operate against those logical tables.
If you take this approach, you will need to create a trigger for each table you wish to monitor in this fashion; we've had a similar need to track changes (at a more granular level), we didn't find a "pseudotable" that corresponds to all tables in a schema/database. You should also be aware that locking semantics will come into play by doing this, as you will have triggers from multiple tables all targeting the same row for an update as part of separate operations -- depending on the concurrency model in effect, you could be looking at performance consequences by doing so if you expect multiple DML queries to operate concurrently against your database.
I would suggest checking Arvo's commented link above for suitability instead; querying system views is more likely to avoid contention (and other performance-related) issues from using triggers in your scenario.
After a bunch more digging on the internet and keep finding opposite results of what I was looking for, and a bunch of trial and error, I found a solution.
First and foremost: having triggers not being dependent on a table (aka, the trigger activates for every table is impossible, it cannot be done, which is too bad, it would've been nice to keep this out of the program code, but nothing I can do about it.
Second: the issue for updates on not being column specific was an error due to my part for searching for triggers not being dependent on specific columns only giving me examples for triggers that are.
The following solution works for MySQL, I have yet to test this on SQL Server, but I expect it to not be too different.
CREATE TRIGGER [tablename]_last_modified_insert
AFTER INSERT/UPDATE/DELETE ON [db].[tablename]
FOR EACH ROW
BEGIN
INSERT INTO [db].last_modified(last_modified_on)
VALUES(current_timestamp())
END
As for dynamically creating these triggers, the following show how I get it to work:
First Query:
SHOW TABLES
I run the above query to get all the tables in the database, exclude the last_modified I made myself, and loop through all of them, creating 3 triggers for each.
Perhaps you could use Audit for SQL Server:
CREATE SERVER AUDIT [ServerAuditName]
TO FILE
(
FILEPATH = N'C:\Program Files......'
)
ALTER SERVER AUDIT [ServerAuditName] WITH (STATE=ON)
GO
CREATE DATABASE AUDIT SPECIFICATION [mySpec]
FOR SERVER AUDIT [ServerAuditName]
ADD (INSERT, UPDATE, DELETE ON DATABASE::databasename BY [public])
WITH (STATE=ON)
GO
Then you can query for changes:
SELECT *
FROM sys.fn_get_audit_file ('C:\Program Files......',default,default);
GO
I need when I create a new table, insert it with the name of the table in another already existing table. I had planned to use a trigger, but I can not find documentation on how to do this. Do you have any idea?
Thank you!
There's nothing in MySQL allowing the definition of a trigger to fire upon table creation, sorry to say.
A trigger is something that happens automatically based on an event in the database. Typically speaking, it’s not a good idea to be creating tables automatically like this. What was your thinking around wanting a table to be created automatically by an event? Normally triggers would be adding or changing individual rows in a DB.
Documentation for mysql triggers can be found at https://dev.mysql.com/doc/refman/5.7/en/trigger-syntax.html
I have a table with 120 columns. I need to set up audit trail which would log any column if it was changed. As it is now, I guess I have to set up a trigger with condition something like this for every column:
IF(NEW.columnName != OLD.columnName)
THEN //log the old value
This would need to be done 120 times... While I would have accepted this approach 20 years ago, today I refuse to believe it's impossible to automate such a simple procedure finding changed columns automatically.
This is what I discovered so far:
Neither NEW nor OLD is a table, it's a sort of a language construct, therefor you can't do "SELECT NOW.*" or something similar.
Dynamic SQL is not allowed in triggers (this could have solved the problem).
Procedures using dynamic SQL are not allowed in triggers (seriously, Oracle, it looks like you have worked really hard to disable this feature no matter what).
I was thinking to use BEFORE and AFTER triggers in conjunction with temporary tables and variables which would have possibly solved the problem, however yet again dynamic SQL would be required. I feel like I hit a dead end.
Is there a solution to this at all?
A side question: would this be possible in PostgreSQL?
UPDATE: I found 2 potential solutions however neither of them look clear enough to me:
using EVENTS as a workaround to use triggers in conjunction with dynamic SQL workaround. I have to admit, I don't quite get this, does this mean that EVENT fires every second no matter what?
This article says that it is possible to use dynamic SQL inside trigger as long as temporary table is used with it. That is still using dynamic SQL, so I don't quite understand.
interesting, I was facing the same problem couple of years ago with implementing dynamic trigger-based audit log. The solution I came up with was to simply generate the SQL trigger code which then can be (automatically) applied to replace old trigger definitions. If memory serves, I created few SQL templates which were processed by a PHP script which in turn was outputting complete trigger definitions based on "SELECT COLUMN_NAME FROM information_schema.COLUMNS WHERE ..." Yes, the trigger code was huge, but it worked! Hope that helps a little =)
i did this for one of the projects by creating a shadow table. if you are not dealing with millions of updates, this might work
when the user logs in, SET #user_id = { logged in user id }
create a trigger on the table before update to copy the row to be modified to a shadow table with the same structure ( note that you cannot have a primary key in the shadow table nor unique keys )
add additional columns to the shadow table ( modified_by, modified_on )
create a small php script to show the diff between columns - this way you dont touvh the existing php code base
if you are dealing with lots of updates and want to keep the shadow table small, a cron can be written to parse the shadow table and identify which column changed and only store this info to another table
I need the sample program in Java for keeping the history of table if user inserted, updated and deleted on that table. Can anybody help in this?
Thanks in advance.
If you are working with Hibernate you can use Envers to solve this problem.
You have two options for this:
Let the database handle this automatically using triggers. I don't know what database you're using but all of them support triggers that you can use for this.
Write code in your program that does something similar when inserting, updating and deleting a user.
Personally, I prefer the first option. It probably requires less maintenance. There may be multiple places where you update a user, all those places need the code to update the other table. Besides, in the database you have more options for specifying required values and integrity constraints.
Well, we normally have our own history tables which (mostly) look like the original table. Since most of our tables already have the creation date, modification date and the respective users, all we need to do is copy the dataset from the live table to the history table with a creation date of now().
We're using Hibernate so this could be done in an interceptor, but there may be other options as well, e.g. some database trigger executing a script, etc.
How is this a Java question?
This should be moved in Database section.
You need to create a history table. Then create database triggers on the original table for "create or replace trigger before insert or update or delete on table for each row ...."
I think this can be achieved by creating a trigger in the sql-server.
you can create the TRIGGER as follows:
Syntax:
CREATE TRIGGER trigger_name
{BEFORE | AFTER } {INSERT | UPDATE |
DELETE } ON table_name FOR EACH ROW
triggered_statement
you'll have to create 2 triggers one for before the operation is performed and another after the operation is performed.
otherwise it can be achieved through code also but it would be a bit tedious for the code to handle in case of batch processes.
You should try using triggers. You can have a separate table (exact replica of your table of which you need to maintain history) .
This table will then be updated by trigger after every insert/update/delete on your main table.
Then you can write your java code to get these changes from the second history table.
I think you can use the redo log of your underlying database to keep track of the operation performed. Is there any particular reason to go for the program?
You could try creating say a List of the objects from the table (Assuming you have objects for the data). Which will allow you to loop through the list and compare to the current data in the table? You will then be able to see if any changes occurred.
You can even create another list with a object that contains an enumerator that gives you the action (DELETE, UPDATE, CREATE) along with the new data.
Haven't done this before, just a idea.
Like #Ashish mentioned, triggers can be used to insert into a seperate table - this is commonly referred as Audit-Trail table or audit log table.
Below are columns generally defined in such audit trail table : 'Action' (insert,update,delete) , tablename (table into which it was inserted/deleted/updated), key (primary key of that table on need basis) , timestamp (the time at which this action was done)
It is better to audit-log after the entire transaction is through. If not, in case of exception being passed back to code-side, seperate call to update audit tables will be needed. Hope this helps.
If you are talking about db tables you may use either triggers in db or add some extra code within your application - probably using aspects. If you are using JPA you may use entity listeners or perform some extra logic adding some aspect to your DAO object and apply specific aspect to all DAOs which perform CRUD on entities that needs to sustain historical data. If your DAO object is stateless bean you may use Interceptor to achive that in other case use java proxy functionality, cglib or other lib that may provide aspect functionality for you. If you are using Spring instead of EJB you may advise your DAOs within application context config file.
Triggers are not suggestable, when I stored my audit data in file else I didn't use the database...my suggestion is create table "AUDIT" and write java code with help of servlets and store the data in file or DB or another DB also ...
I've been hearing about triggers, and I have a few questions.
What are triggers?
How do I set them up?
Are there any precautions, aside from typical SQL stuff, that should be taken?
Triggers allow you to perform a function in the database as certain events happen (eg, an insert into a table).
I can't comment on mysql specifically.
Precaution: Triggers can be very alluring, when you first start using them they seem like a magic bullet to all kinds of problems. But, they make "magic" stuff happen, if you don't know the database inside out, it can seem like really strange things happen (such as inserts into other tables, input data changing, etc). Before implementing things as a trigger I'd seriously consider instead enforcing the use of an API around the schema (preferably in the database, but outside if you can't).
Some things I'd still use triggers for
Keeping track of "date_created" and "date_last_edited" fields
Inserting "ID"'s (in oracle, where there is no auto id field)
Keeping change history
Things you wouldn't want to use triggers for
business rules/logic
anything which connects outside of the database (eg a webservice call)
Access control
Anything which isn't transactional ( anything you do in the trigger MUST be able to rollback with the transaction )
From dev.mysql.com, a trigger is
...a named database object that is
associated with a table and that is
activated when a particular event
occurs for the table.
The syntax to create them is also documented at that site.
Briefly,
CREATE
[DEFINER = { user | CURRENT_USER }]
TRIGGER trigger_name trigger_time trigger_event
ON tbl_name FOR EACH ROW trigger_stmt
And they provide an example:
CREATE TABLE account (acct_num INT, amount DECIMAL(10,2));
CREATE TRIGGER ins_sum BEFORE INSERT ON account FOR EACH ROW SET #sum = #sum + NEW.amount;
You at least need to abide by all the restrictions on stored functions.
You won't be able to lock tables, alter views, or modify the table that triggered the trigger. Also triggers may cause replication problems.
A trigger is a named database object that is associated with a table and that is activated when a particular event occurs for the table.
To create a trigger:
CREATE TRIGGER triggerName [BEFORE|AFTER] [INSERT|UPDATE|DELETE|REPLACE] ON tableName FOR EACH ROW SET stuffToDoHERE;
Even though I answered this part the other question still stands.
This question is old and other answers are very good, but since the user asked about precautions that should be taken, I want to add something:
If you use replication in a complex environment, don't make a massive use of Triggers, and don't call stored procedures from triggers.
Triggers are slow in MySQL.
You can't use some SQL statements within triggers. And some statements are permitted but should be avoided, like LOCK. The general rule is: if you don't fully understand the implications of what you are doing, you shouldn't do it.
Triggers can cause endless loops, so be careful.