How can I edit values of an INSERT in a trigger on SQL Server?

Question

I have the table Tb

ID | Name   | Desc
-------------------------
 1 | Sample | sample desc

I want to create a trigger on INSERT that will change the value of the inserting Desc, for example:

INSERT INTO Tb(Name, Desc) VALUES ('x', 'y')

Will result in

ID | Name   | Desc
-------------------------
 1 | Sample | sample desc
 2 | x      | Y edited

In the above example I got the value of the inserting Desc changed it to uppercase and added edited on the end.

That's what I need, get the Desc that is being inserted and modify it.

How can I do that?

Is it better to handle it after the insert with an update? Or make a trigger with INSTEAD OF INSERT and modify it everytime the table structure changes?

Solution 1

Use an after insert trigger. Join from the inserted pseudo table to Tb on the primary key. Then update the values of desc. Something like: (But may not compile)

CREATE TRIGGER TbFixTb_Trg 
ON  Tb  
AFTER INSERT 
AS  
BEGIN 
    UPDATE Tb
    SET DESC = SomeTransformationOf(i.DESC)
    FROM Tb
    INNER JOIN inserted i on i.Id = Tb.Id
END  
GO

This trigger happens after the insert has happened, but before insert statement completes. So the new, incorrect values are already placed in the target table. This trigger will not need to change as columns are added, deleted, etc.

Caveat Integrity constraints are enforced before the after trigger fires. So you can't put on a check constraint to enforce the proper form of DESC. Because that would cause the statement to fail prior to the trigger having a chance to fix anything. (Please double check this paragraph before relying on it. It's been awhile since I've written a trigger.)

Solution 2

I'm not sure where your going to get the actual new value for desc, but I assume you are getting it from another table or some such. But you probably have a reason for wanting to do it this way so below is an example of how I would go about it.

What you want is called an INSTEAD OF INSERT trigger, it fire instead of an insert on the table with what every logic you give it.

CREATE TRIGGER trgUpdateDesc
ON  Tb 
INSTEAD OF INSERT
AS 
BEGIN
    SET NOCOUNT ON;
    INSERT INTO Tb (Name, [Desc])
    SELECT Name, [Desc] + 'edited'
    FROM inserted
END 
GO

I've hard coded the word 'edited' in there as I'm not sure where you want to get the value, but you can easily replace that with a variable or a value from another table.

Oh also be sure the put the [] around Desc, as it is a key word in sql server (stands for descending)

Hope that helps!

Edit:

If you want to make it a little more robust so that it doesn't depend on the table structure as much you could use an AFTER INSERT trigger to just updates that field like so.

CREATE TRIGGER [dbo].[trgUpdateDesc]
   ON  [dbo].[Tb] 
   AFTER INSERT
AS 
BEGIN
    SET NOCOUNT ON;
    UPDATE Tb
    SET [Desc] = UPPER(inserted.[Desc]) +  ' Edited'
    FROM inserted INNER JOIN Tb On inserted.id = Tb.id
END 

Solution 3

Temp table can help to use INSTEAD OF INSERT trigger while avoiding to explicitly listing all unrelated table columns:

CREATE TRIGGER trgUpdateDesc
    ON Tb 
    INSTEAD OF INSERT
AS 
BEGIN
    SET NOCOUNT ON;
    select * into #tmp from inserted;
    UPDATE #tmp SET Desc = Desc + 'edited' --where ...;
    insert into Tb select * from #tmp;
    drop table #tmp;
END 

This code will not need to be fixed when new columns are added to the table.

But beware of some additional overhead of temp tables.

Also note that SQL Server triggers fire once for bulk DML operations and must correctly handle multi-row inserts.

Solution 4

You may want to look at INSTEAD OF triggers.

CREATE TRIGGER Tb_InsteadTrigger on Tb
INSTEAD OF INSERT
AS
BEGIN
...

This will allow you to manipulate the data before it goes into the table. The trigger is responsible for inserting the data to the table.

View more solutions

Comments

[Claudio Redi]

Can't you do this in a different way? I personally hate when I see weird things happening on the data and I finally realize that a ghost trigger was doing some stuff

[BrunoLM]

I will use this to fix a problem that comes from a code I don't have access. It inserts the url of an item, but it doesn't remove accents and special characters, I will make a trigger do that for me.

[Jim]

This isn't such a hard concept and it boggles my mind that Microsoft can't provide such simple functionality like in Oracle where you can modify any values before insert by editing the :NEW values. i.e. if I want to always cast usernames to upper before inserting them (assuming I can't just do it in code for some reason), you could have a before insert trigger that does :NEW.username := UPPER(:NEW.username). In SQL Server though, you can't do the equivalent with values in the INSERTED temp table, and that really sucks.

[Triynko]

Yeah, you can. In the instead of trigger, just run an insert statement that selects all the columns from the inserted table into the real table, but modify the select for the column you want to make upper. For example, in the instead of insert trigger, just run a statement like insert into Table (a,b,c) select a, ToUpper(b), c from inserted. Microsoft tech blows oracle out of the water.

[BrunoLM]

I want to get the value of the inserted Desc and modify it. I should have said that. I will update my question, sorry for that

[BrunoLM]

What if my table changes? I will have to update the trigger everytime? Isn't there another way to do that?

[Lukas Eder]

Also, this solution won't make the actually inserted values available to the OUTPUT clause!

[CodenameCain]

What is "Trgt" in this context?

[Shannon Severance]

@CodenameCain A mistake. Should be Tb or the from clause needs to be from Tb Trgt. thanks for catching that.

[Anders Lindén]

This produces the following error: An explicit value for the identity column in table 'Tb' can only be specified when a column list is used and IDENTITY_INSERT is ON.

[Vadzim]

@Anders, I guess, this approach may be incompatible with IDENTITY columns. But you may also try to drop this column in #tmp table inside trigger.

[Fandango68]

But be careful because INSTEAD assumes you're in charge of all the columns in the table.

[Fandango68]

The OP was about updating a column value after the insert is triggered. Please elaborate your answer because it looks like it has nothing to do with the question asked.