T-SQL insert trigger - sql-server

Question

The database I am working on has the following insert trigger. The problem is the trigger will not work with a multi-row insert. When I try and insert more than one row at a time I get the following message.
Msg 512, Level 16, State 1, Procedure _ti_UD06, Line 8
Subquery returned more than 1 value. This is not permitted when the subquery
follows =, !=, <, <= , >, >= or when the subquery is used as an
expression. I think the problem is with the following statement it
will return more than one value when multiple rows are inserted.
How can I make this insert trigger work with multiple rows?
IF(SELECT PROGRESS_RECID FROM inserted) IS NULL
There are 2 columns in the table, PROGRESS_RECID and PROGRESS_RECID_IDENT_.
I think the PROGRESS_RECID_IDENT_ is an Identity column for SQL Server. The PROGRESS_RECID_INDENT is used by the PROGRESS Server.
If a new record is inserted the trigger checks to see if the PROGRESS_RECID is null and will set it to the inserted.Identity
ALTER trigger [dbo].[_ti_UD06] ON [dbo].[UD06] for insert as
begin
if ( select PROGRESS_RECID from inserted) is NULL
begin
update t set PROGRESS_RECID = i.IDENTITYCOL
from UD06 t JOIN INSERTED i ON
t.PROGRESS_RECID_IDENT_ = i.PROGRESS_RECID_IDENT_
select convert (bigint, ##identity)
end
end
Here is the Table definition
CREATE TABLE [dbo].[UD06](
[Company] [nvarchar](8) NULL,
[Key1] [nvarchar](50) NOT NULL,
[Key2] [nvarchar](50) NULL,
[Key3] [nvarchar](50) NULL,
[Key5] [nvarchar](50) NULL,
[Character01] [nvarchar](max) NULL,
[Character02] [nvarchar](max) NULL,
[Character03] [nvarchar](max) NULL,
[Character04] [nvarchar](max) NULL,
[Character05] [nvarchar](max) NULL,
[Character06] [nvarchar](max) NULL,
[Character07] [nvarchar](max) NULL,
[Character08] [nvarchar](max) NULL,
[Character09] [nvarchar](max) NULL,
[Character10] [nvarchar](max) NULL,
[SysRowID] [nvarchar](36) NULL,
[SysRevID] [int] NULL,
[BitFlag] [int] NULL,
[GlobalUD06] [tinyint] NULL,
[GlobalLock] [tinyint] NULL,
[PROGRESS_RECID] [bigint] NULL,
[PROGRESS_RECID_IDENT_] [bigint] IDENTITY(1,1) NOT NULL
) ON [PRIMARY]

Answer 1

I think you're pretty close already:
ALTER trigger [dbo].[_ti_UD06] ON [dbo].[UD06] for insert as
begin
UPDATE t
SET PROGRESS_RECID = t.PROGRESS_RECID_IDENT_ --not "IDENTITYCOL"
FROM UD06 t
JOIN inserted i ON t.PROGRESS_RECID_IDENT_ = i.PROGRESS_RECID_IDENT_
WHERE i.PROGRESS_RECID is Null
-- Not sure what this bit is for? I doubt it's a good idea though...
select convert (bigint, ##identity)
end