Question

quick question, I am building a simple trigger which purpose is to decrement the value of a table field called openSeats, the trigger is executing on insert but I dont know what commands to use to say:
Decrement the value openSeats , where Id is equal to inserted Id

USE [Training]
GO
/****** Object:  Trigger [dbo].[DecrementSeat]    Script Date: 11/04/2011 14:55:17 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
-- =============================================
-- Author:      <Author,,Name>
-- Create date: <Create Date,,>
-- Description: <Description,,>
-- =============================================

ALTER TRIGGER [dbo].[DecrementSeat]
   ON  [dbo].[personTraining]
   AFTER INSERT

AS 
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;

    UPDATE [dbo].[tbl_training]
    SET openSeats = openSeats - 1 
    WHERE training_id = 

END

Answer 1

Try this in your trigger:

update dbo.tbl_training
set openSeats = openSeats - 1
where training_id in
(
    select training_id
    from inserted
)

It utilizes the inserted dynamic table that SQL Server populates with inserted data during a trigger.