Is it possible to mark a column immutable in MSSQL?
Seems like it would be a useful DDL feature; once a value is set in a row ('row' being defined as a specific relation of values to a primary key), it could not be changed without deletion of the row.
Obviously (like most things) this is more than doable in the application layer, but half the fun of SQL DDL is error-checking your application code.
If the user doing the DML is not the owner of the objects and not "db_owner" in the database itself, you can just grant "insert" privilege, but not update privilege for that table:
Assuming a table with id, col1, col2
grant insert, select, delete on the_table to the_user;
grant update (id, col2) on the_table to the_user;
With these grants the_user can insert rows and supply values for all three columns. He can also update the id and the col2 column, but not the col1 column.
The db_owner (and possibly the creator/owner of the table) can always update all columns. I don't know if there is a way to revoke that privilege from those rolws.
It's possible, using an UPDATE TRIGGER like this:
CREATE TRIGGER trgAfterUpdateAsset ON dbo.Asset
FOR UPDATE AS
IF UPDATE(AssetTypeID) AND EXISTS (SELECT * FROM inserted i JOIN deleted d ON i.ID = d.ID WHERE i.AssetTypeID <> d.AssetTypeID)
BEGIN
RAISERROR ('AssetTypeID cannot change.', 16, 1);
ROLLBACK TRAN
END
(Note: The table has a Primary Key column, called ID).
I'm only rejecting the update if the value of AssetTypeID changes. So the column could be present in an update, and if it specified the old value, than it would pass through. (I needed this way)
No, there is no such feature in SQL Server.
The closest I can think about is an update trigger on the table that checks if the values in the specific column are the same for the INSERTED and DELETED logical tables and rejects the updates for the changed rows.
To my knowledge, this is not possible with DDL. However, you could implement BEFORE UPDATE triggers to meet your requirement. In the BEFORE UPDATE trigger, you could raise an exception or do whatever you want rather than update the row.
Another approach is to deny update rights to the table and create a stored procedure (which users do have the right to execute) that does not update the immutable field.
Related
Is there any way in Firebird to execute an UPDATE instead a DELETE through a trigger?
This is possible in Microsoft SQL Server by declaring the triggers as "INSTEAD".
The problem is that we have an application that uses a Firebird database and we want to prevent the deletes of records and mark then as "deleted" (a new field), but without showing any error to the user, and "cheating" the app.
You cannot do this with tables, but you can do it with views. Views can have triggers on insert, update and delete that modify the underlying table(s). See also Updatable Views in the Firebird 2.5 Language Reference.
In short, create a table for the data, add a view, add triggers that insert/update/delete through the view to the underlying table. Users can then use the view as if it is a table.
An example
I'm using Firebird 3, but this will work with minor modifications in Firebird 2.5 and earlier.
A table example_base:
create table example_base (
id bigint generated by default as identity constraint pk_example_base primary key,
value1 varchar(100),
deleted boolean not null default false
)
A view example:
create view example (id, value1)
as
select id, value1
from example_base
where not deleted;
Do not create the view with with check option, as this will disallow inserts as the absence of the deleted column in the view will prevent Firebird from checking the invariant.
Then add an insert trigger:
create trigger tr_example_insert before insert on example
as
begin
if (new.id is not null) then
-- Don't use identity
insert into example_base(id, value1) values (new.id, new.value1);
else
-- Use identity
-- mapping generated id to new context
-- this way it is available for clients using insert .. returning
insert into example_base(value1) values (new.value1)
returning id into :new.id;
end
The above trigger ensures the 'by default as identity' primary key of the underlying table is preserved, and allows insert into example .. returning to report on the generated id.
An update trigger
create trigger tr_example_update before update on example
as
begin
-- Consider ignoring modification of the id (or raise an exception)
update example_base
set id = new.id, value1 = new.value1
where id = old.id;
end
The above trigger allows modification of the primary key; you may want to consider just ignoring such a modification or even raising an exception.
And finally a delete trigger:
create trigger tr_example_delete before delete on example
as
begin
update example_base
set deleted = true
where id = old.id;
end
This trigger will mark the record in the base table as deleted.
To use this, just grant your users select, insert and update privileges to the view (and not the table).
The only caveat I'm aware of is that defining foreign keys will need to point to example_base, not to example, and the behavior of foreign keys will be slightly off. The record in the base table will continue to exist, so the foreign key will not block deletion. If that is something that is necessary, you will need to emulate constraint behavior (which could be tricky).
YES! It can be made on VIEWs.
That's the way I solved it.
If a View has a trigger, then the trigger is the responsible of making the real update or delete on the underlying table.... So... a DELETE trigger that makes an UPDATE to the table solved my problem.
I've a table. In this table I have two columns - 'insert_name' and 'modified_name'. I need to insert into this columns data about who has inserted data into the table('insert_name') and who has changed these data in the table (modified_name). How it can be done?
You are looking for basic DML statements.
If your record is already in the table, then you need to UPDATE it. Otherwise, when you are about to add your record to it and it doesn't already exist in the destination table then you are looking for INSERT INTO statement.
Example of updating information for record with first id:
UPDATE yourtable SET insert_name = 'value1', modified_name = 'value2' WHERE id = 1
Example of inserting new record:
INSERT INTO yourtable(id, company_name, product_name, insert_name)
VALUES (1, 'Google', 'PC', 'value1')
If you are looking for automatic changes to those columns then you need to look into triggers.
Remember that more often than not you may find that the application connecting to the database is using single database user in which case you probably know the context within the application itself (who inserts, who updates). This does eliminate triggers and put the task straight on simple insert/update commands from within your application layer.
You might be able to use the CURRENT_USER function to find the name of the user making the change.
The value from this function could then be used to update the appropriate column. This update could be done as part of the INSERT or UPDATE statement. Alternatively use an INSERT or UPDATE trigger.
Personally I avoid triggers if I can.
For those 2 columns add Current_User as Default constraint.
As the first time Insert Statement will save them with current login user names. For update write an Update trigger with the same Current_User statement for the column Modified_Name.
If and only if your application business logic can't update the column modified_nme then only go for Trigger.
See the use of Current_Use
https://msdn.microsoft.com/en-us/library/ms176050.aspx
Suppose I have tables T1 and T2
Columns of T1 -->Value
Columns of T2 -->OldValue NewValue
What I require is a trigger which will insert a record in T2 on updation of T1 , I need to know the old value and new value also , I have never used triggers before , so can any help me with this , how do I go about creating this trigger.Is it possible ,thanks.
Well, you start writing a trigger with CREATE TRIGGER:
CREATE TRIGGER NameOfTheTriggerPlease
…
The table that should trigger the additional action is T1 so the trigger should be defined ON that table:
CREATE TRIGGER T1OnUpdate /* that's just an example,
you can use a different name */
ON T1
…
The action that the trigger should be invoked on is UPDATE and the timing is AFTER the update, so…
CREATE TRIGGER T1OnUpdate
ON T1
AFTER UPDATE
…
Now's the time to introduce the body of the trigger, i.e. the statements that should actually be executed by the trigger. You introduce the body with the AS keyword followed by the statements themselves.
In your case, there would be just one statement, INSERT, which is obvious. What's not so obvious is how we are going to access the old and the new values. Now, SQL Server offers you two virtual tables, INSERTED and DELETED, and you can easily guess that the former contains all the new values and the latter the old ones.
These tables have the same structure as the table the trigger is assigned to, i.e. T1. They only contain rows that were affected by the particular UPDATE statement that invoked the trigger, which means there may be more than one. And that, in turn, means that you need to have some primary key or a unique column (or a set of columns) in your T1 table that you can use in the trigger to match deleted and inserted rows. (In fact, you might also need your T2 table to have a column that would reference the T1's primary key, so you could later establish which row of T1 had which values stored in T2.)
For the purposes of this answer, I'm going to assume that there's a primary key column called PK and a foreign key column of the same name in T2. And the INSERT statement then might look like this:
CREATE TRIGGER T1OnUpdate
ON T1
AFTER UPDATE
AS
INSERT INTO T2 (PK, OldValue, NewValue)
SELECT i.PK, i.Value, d.Value
FROM INSERTED i INNER JOIN DELETED d ON i.PK = d.PK
One last (but not least) thing to remember: the entire CREATE TRIGGER statement should be the only one in the batch, i.e. there should be no statements preceding the CREATE TRIGGER keywords (but you can put comments there) and, likewise, everything after the AS keyword is considered part of the trigger's body (but you can put the GO delimiter to indicate the end of the statement if you are running the script in SQL Server Management Studio, for instance).
Useful reading:
CREATE TRIGGER (Transact-SQL)
I'm not going to build the whole thing for you (no fun, right?) but I can point you in the right direction
create trigger logUpdate
on T1
After update
as
begin
insert into T2...
--here is just an example
select * from deleted --the DELETED table contains the OLD values
select * from inserted --the INSERTED table contains the NEW values
end
remember that DELETED and INSERTED are internal tables that contains old and new values. On a update trigger, they both exist. On a insert trigger, DELETED will be null because there is nothing being delete. Same logic on a delete trigger, the INSERTED will be empty
EDIT:
answering your question: no matter how many fields you update, your DELETED and INSERTED tables you have all the columns of all the rows affected. Of course, if you update only one column, all the other will have the same value on DELETED and INSERTED
create trigger T_UPD_T1
on T1 FOR update
as
insert into T2 select deleted.value, inserted.value from inserted, deleted
We have a status table. When the status changes we currently delete the old record and insert a new.
We are wondering if it would be faster to do a select to check if it exists followed by an insert or update.
Although similar to the following question, it is not the same, since we are changing individual records and the other question was doing a total table refresh.
DELETE, INSERT vs UPDATE || INSERT
Since you're talking SQL Server 2008, have you considered MERGE? It's a single statement that allows you to do an update or insert:
create table T1 (
ID int not null,
Val1 varchar(10) not null
)
go
insert into T1 (ID,Val1)
select 1,'abc'
go
merge into T1
using (select 1 as ID,'def' as Val1) upd on T1.ID = upd.ID --<-- These identify the row you want to update/insert and the new value you want to set. They could be #parameters
when matched then update set Val1 = upd.Val1
when not matched then insert (ID,Val1) values (upd.ID,upd.Val1);
What about INSERT ... ON DUPLICATE KEY? First doing a select to check if a record exists and checking in your program the result of that creates a race condition. That might not be important in your case if there is only a single instance of the program however.
INSERT INTO users (username, email) VALUES ('Jo', 'jo#email.com')
ON DUPLICATE KEY UPDATE email = 'jo#email.com'
You can use ##ROWCOUNT and perform UPDATE. If it was 0 rows affected - then perform INSERT after, nothing otherwise.
Your suggestion would mean always two instructions for each status change. The usual way is to do an UPDATE and then check if the operation changed any rows (Most databases have a variable like ROWCOUNT which should be greater than 0 if something changed). If it didn't, do an INSERT.
Search for UPSERT for find patterns for your specific DBMS
Personally, I think the UPDATE method is the best. Instead of doing a SELECT first to check if a record already exists, you can first attempt an UPDATE but if no rows are affected (using ##ROWCOUNT) you can do an INSERT.
The reason for this is that sooner or later you might want to track status changes, and the best way to do this would be to keep an audit trail of all changes using a trigger on the status table.
I wish to make a trigger but i'm not sure how to grab the data for whatever caused the trigger.
I have a simlpe table.
FooId INT PK NOT NULL IDENTITY
Name VARCHAR(100) NOT NULL
I wish to have a trigger so that when an UPDATE, INSERT or DELETE occurs, i then do the following.
Pseduocode
IF INSERT
Print 'Insert' & Name
ELSE IF UPDATE
Print 'Update' & FooId & Name
ELSE IF DELETE
Print 'Delete' & FooId & Name
Now, I know how to make a trigger for a table.
What i don't know how to do is figure out the values based on what the trigger type is.
Can anyone help?
Edit: Not sure if it helps, but db is Sql Server 2008
the pseudo table "inserted" contains the new data, and "deleted" table contains the old data.
You can do something like
create trigger mytrigger on mytable for insert, update, delete
as
if ( select count(*) from inserted ) > 0
-- insert or update
select FooId, Name from inserted
else
-- delete
select FooId, Name from deleted
To clarify all the comments made by others, on an insert, the inserted table contains data and deleted is empty. On a delete, the situation is reversed. On an update, deleted and inserted contain the "before" and "after" copy of any updated rows.
When you are writing a trigger, you have to account for the fact that your trigger may be called by a statement that effects more than one row at a time.
As others have pointed out, you reference the inserted table to get the values of new values of updated or inserted rows, and you reference the deleted table to get the value of deleted rows.
SQL triggers provide an implicitly-defined table called "inserted" which returns the affected rows, allowing you to do things like
UPDATE mytable SET mytimestamp = GETDATE() WHERE id IN (SELECT id FROM inserted)
Regarding your code sample, you'll want to create separate INSERT, UPDATE and DELETE triggers if you are performing separate actions for each.
(At least, this is the case in SQL Server... you didn't specify a platform.)
On 2008, there is also MERGE command. How do you want to handle it?
Starting from 2008, there are four commands you can modify a table with:
INSERT, UPDATE, DELETE, and MERGE:
http://blogs.conchango.com/davidportas/archive/2007/11/14/SQL-Server-2008-MERGE.aspx
http://sqlblogcasts.com/blogs/grumpyolddba/archive/2009/03/11/reasons-to-move-to-sql-2008-merge.aspx
What do you want your trigger to do when someone issues a MERGE command against your table?