How to use nested If statements in SQL trigger - sql

Question

I'm trying to learn SQL triggers to automatically handle events in my database but I'm having some problems with execution.
If I run the following code:
declare #userid numeric(18,0);
declare #username nvarchar(max);
set #userid = 400
execute GetUserNameFromID #userid,#username output
select #username
which calls the following stored procedure:
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE GetUserNameFromID
-- Add the parameters for the stored procedure here
#UserID numeric(18,0),
#UserName nvarchar(MAX) OUT
AS
BEGIN
SET NOCOUNT ON;
SELECT #UserName = u.name from Users u where ID=#UserID
END
GO
I get a nice result 'sometestuser'
But when calling it from my trigger it fails to return a value from the stored procedure:
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER Trigger [dbo].[CheckIfUserHasNoItemsLeft] on [dbo].[Items] for update
As
Begin
set nocount on
declare #inactive_user nvarchar(50);
declare #userid numeric(18,0);
declare #username nvarchar(MAX);
if ( select Count(*) from inserted ) > 1 RaIsError( 'CheckIfIserHasNoItemsLeft: No more than one row may be processed.', 25, 42 ) with log
if update(InactiveUser)
set #inactive_user = (select InactiveUser from inserted)
if #inactive_user is not null
set #userid = (select CID from inserted)
execute GetuserNameFromID #userid,#username output
if #username is not null
insert into tasks (Task) values ('The last item for ' + #username + ' has been marked inactive, check if this user should now be also marked inactive.')
End
InactiveUser is the name of the app user who has marked this item inactive, it is what I am using as a check to see if the item has been set inactive rather than create an additional boolean column just for this purpose.
I'm sure it's something simple but information on If...Then statements for SQL seems to be limited and a lot of answers suggest using Case but the query editor gives me errors about incorrect syntax no matter which way I try to do it that way.
As I'm learning I'm more than happy for someone to show me a completely new way of handling this if what I've done is wrong or bad design. I'm hoping to create a set of triggers that will add items to the tasks table for administrators to check when user accounts appear to be stale and other maintenance checks etc.
I am using SQL server 2005.
Thank you.
Edit: Changed 'value <> null' to 'value is not null'
Edit2: Added HABO's suggestion to throw an error if more than one row is detected.

Answer 1

How about we take a whole new approach to this. Processes like this are exactly why the inline table valued functions were created.
Let's start by converting your stored procedure to an inline table valued function.
CREATE FUNCTION GetUserNameFromID
(
#UserID numeric(18,0)
) RETURNS TABLE
AS RETURN
SELECT u.name
from Users u
where ID = #UserID
GO
That is a LOT simpler and cleaner than that stored procedure with an output variable.
Here is where it really starts to make a difference. Here is what you could do with that trigger using the newly created iTVF.
ALTER Trigger [dbo].[CheckIfUserHasNoItemsLeft] on [dbo].[Items] for update
As Begin
set nocount on
if update(InactiveUser)
insert into tasks (Task)
select 'The last item for ' + u.name + ' has been marked inactive, check if this user should now be also marked inactive.'
from inserted i
cross apply dbo.GetUserNameFromID(i.CID) u
end
This is super simple AND it is fully set based so if you update 1 or 1,000 rows it will work correctly.