Easy Trigger question - sql-server

Question

I would imagine that this would be an easy question for someone who works a lot with T-SQL and especially Triggers:
I want to enforce the following constraints on all updates and inserts to this table:
If DiscountTypeId = 1, then FlatFee must not be NULL.
If DiscountTypeId = 2, then DiscountRate must not be null.
If either one of these two conditions fail on an insert or update to the table, I'd like to return an appropriate error.
The trigger appears not to do anything yet. .Can you provide the necessary changes so it performs as described?
USE [PandaVisa2008]
GO
/****** Object: Table [dbo].[CustomerSpeed] Script Date: 11/04/2010 15:51:10 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[CustomerSpeed](
[CustomerSpeedId] [int] NOT NULL,
[CustomerId] [int] NULL,
[SpeedId] [int] NOT NULL,
[DiscountTypeId] [int] NOT NULL,
[FlatFee] [money] NULL,
[DiscountRate] [decimal](3, 3) NULL,
CONSTRAINT [PK_AgentFee] PRIMARY KEY CLUSTERED
(USE [PandaVisa2008]
GO
/****** Object: Trigger [dbo].[TRG_CustomerSpeed_OnInsertUpdate] Script Date: 11/04/2010 15:38:06 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER TRIGGER [dbo].[TRG_CustomerSpeed_OnInsertUpdate]
ON [dbo].[CustomerSpeed]
FOR INSERT, UPDATE
AS
BEGIN
DECLARE #DiscountTypeId INT
DECLARE #FlatFee MONEY
DECLARE #DiscountRate DECIMAL(3, 3)
SELECT
#DiscountTypeId = DiscountTypeId,
#FlatFee = FlatFee,
#DiscountRate = DiscountRate
FROM
inserted
IF #DiscountTypeId = 1
AND #FlatFee IS NULL
BEGIN
RAISERROR (N'If #DiscountTypeId is 1, FlatFee must not be NULL',
10,
1)
END
IF #DiscountTypeId = 2
AND #DiscountRate IS NULL
BEGIN
RAISERROR (N'If #DiscountTypeId is 2, #DiscountRate must not be NULL',
10,
1)
END
END
[CustomerSpeedId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO
ALTER TABLE [dbo].[CustomerSpeed] WITH CHECK ADD CONSTRAINT [CK_CustomerSpeed] CHECK (([DiscountRate]>(0) AND [DiscountRate]<(1)))
GO
ALTER TABLE [dbo].[CustomerSpeed] CHECK CONSTRAINT [CK_CustomerSpeed]
GO
EDIT
I got it to work. I haven't read up on Triggers to remedy my fundamental lack of understanding, but t his seemed to work, although I believe that the Check Constraint is the better approach:
ALTER TRIGGER [dbo].[TRG_CustomerSpeed_OnInsertUpdate]
ON [dbo].[CustomerSpeed]
FOR INSERT, UPDATE
AS
BEGIN
IF EXISTS (SELECT
1
FROM
inserted I
WHERE I.DiscountTypeId = 1
AND I.FlatFee IS NULL)
BEGIN
ROLLBACK TRANSACTION
RAISERROR (N'If DiscountTypeId is 1, FlatFee must not be NULL',
10,
1)
END
IF EXISTS (SELECT
1
FROM
inserted I
WHERE I.DiscountTypeId = 2
AND I.DiscountRate IS NULL)
BEGIN
ROLLBACK TRANSACTION
RAISERROR (N'If DiscountTypeId is 2, DiscountRate must not be NULL',
10,
1)
END
/*
IF #DiscountTypeId = 2
AND #DiscountRate IS NULL
BEGIN
Rollback Transaction
RAISERROR (N'If #DiscountTypeId is 2, DiscountRate must not be NULL',
10,
1)
END
*/
END
Your comments are welcomed.

Answer 1

I'd use a CHECK constraint, not a triggers
ALTER TABLE Mytable WITH CHECK ADD
CONSTRAINT CK_MyTable_GoodName CHECK (
NOT (DiscountTypeId = 1 AND Flatfee IS NULL)
AND
NOT (DiscountTypeId = 2 AND DiscountRate IS NULL)
)
Also, need to consider "if DiscountTypeId <> 1, does Flatfee have to be NULL" etc

Answer 2

You fundamentally do not understand triggers. The very first thing you need to do is go read about triggers in Books Online with particular emphasis on learning about the inserted and deleted psuedotables. Next thing you need to know is a trigger should NEVER be written as if it will handle only one record at a time. Triggers operate on batches of records and trigger code must account for that.

Answer 3

I don't believe triggers can raise errors, problem #1.