MSSql Trigger returns a subquery error

I have the following trigger below which works great in the case of an insert or a single row update but returns the following error

Msg 512, Level 16, State 1, Procedure UPDATE_TSD_DOW, Line 10
Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

when I execute a SQL statement such as "update tsdetail set tsd_min=tsd_min*1

I realize it has to do with the fact that I am trying to update the same row that the trigger is based on but that is what I need.  I also do not think I can use an INSTEAD OF trigger since I won't know what other columns are in the table and my understanding is that the INSTEAD OF replaces the INSERT so I would need to know the columns being inserted.

Is there anoher way to do this in MSSql?  Oracle uses something like ":NEW.TSD_WED :" which you can set in a BEFORE INSERT, UPDATE trigger.  Can I do something like this in MSSql?



SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO

ALTER TRIGGER [dbo].[UPDATE_TSD_DOW] ON [dbo].[TSDETAIL]
AFTER INSERT,UPDATE
AS
SET NOCOUNT ON
DECLARE @v_dow CHAR(3)
DECLARE @TSD_DATE SMALLDATETIME
DECLARE @TSD_KEY NUMERIC(11)

BEGIN
    SET @TSD_DATE=(SELECT CAST(inserted.TSD_DATE AS SMALLDATETIME) FROM inserted)
    SET @TSD_KEY=(SELECT TSD_KEY FROM inserted)
    SET @v_dow = LEFT(UPPER(DATENAME(weekday, @TSD_DATE)),3)

    IF @v_dow = 'MON'
      UPDATE [dbo].[TSDETAIL] SET TSD_MON = [dbo].TSDETAIL.TSD_MIN,
      TSD_TUE = 0,TSD_WED = 0,TSD_THU = 0,TSD_FRI = 0,TSD_SAT = 0,TSD_SUN = 0
      WHERE [dbo].[TSDETAIL].TSD_KEY = @TSD_KEY

    IF @v_dow = 'TUE'
      UPDATE [dbo].[TSDETAIL] SET TSD_TUE = [dbo].TSDETAIL.TSD_MIN,
      TSD_MON = 0,TSD_WED = 0,TSD_THU = 0,TSD_FRI = 0,TSD_SAT = 0,TSD_SUN = 0
      WHERE [dbo].[TSDETAIL].TSD_KEY = @TSD_KEY

    IF @v_dow = 'WED'
      UPDATE [dbo].[TSDETAIL] SET TSD_WED = [dbo].TSDETAIL.TSD_MIN,
      TSD_MON = 0,TSD_TUE = 0,TSD_THU = 0,TSD_FRI = 0,TSD_SAT = 0,TSD_SUN = 0
      WHERE [dbo].[TSDETAIL].TSD_KEY = @TSD_KEY

    IF @v_dow = 'THU'
      UPDATE [dbo].[TSDETAIL] SET TSD_THU = [dbo].TSDETAIL.TSD_MIN,
      TSD_MON = 0,TSD_TUE = 0,TSD_WED = 0,TSD_FRI = 0,TSD_SAT = 0,TSD_SUN = 0
      WHERE [dbo].[TSDETAIL].TSD_KEY = @TSD_KEY

    IF @v_dow = 'FRI'
      UPDATE [dbo].[TSDETAIL] SET TSD_FRI = [dbo].TSDETAIL.TSD_MIN,
      TSD_MON = 0,TSD_TUE = 0,TSD_WED = 0,TSD_THU = 0,TSD_SAT = 0,TSD_SUN = 0
      WHERE [dbo].[TSDETAIL].TSD_KEY = @TSD_KEY

    IF @v_dow = 'SAT'
      UPDATE [dbo].[TSDETAIL] SET TSD_SAT = [dbo].TSDETAIL.TSD_MIN,
      TSD_MON = 0,TSD_TUE = 0,TSD_WED = 0,TSD_THU = 0,TSD_FRI = 0,TSD_SUN = 0
      WHERE [dbo].[TSDETAIL].TSD_KEY = @TSD_KEY

    IF @v_dow = 'SUN'
      UPDATE [dbo].[TSDETAIL] SET TSD_SUN = [dbo].TSDETAIL.TSD_MIN,
      TSD_MON = 0,TSD_TUE = 0,TSD_WED = 0,TSD_THU = 0,TSD_FRI = 0,TSD_SAT = 0
      WHERE [dbo].[TSDETAIL].TSD_KEY = @TSD_KEY
END
GO

SET ANSI_NULLS OFF
GO
SET QUOTED_IDENTIFIER OFF
GO