Make column to always contain calculated value without triggers - sql

Question

I was doing practice tests before 70-461 exam and one of questions was to create table that:
Has 4 columns, saleid, unitprice, amount, and price.
Price is calculated by multiplication of unitprice and amount
You can't use triggers.
The last one killed me. How can I do it without triggers? Here is my solutions using instead of triggers.
CREATE TABLE [HR].[Stuff](
[saleid] [int] IDENTITY(1,1) NOT NULL,
[unitprice] [float] NOT NULL,
[amount] [float] NOT NULL,
[price] [float] NULL,
)
GO
CREATE TRIGGER [calculate_price_insert]
ON [HR].[Stuff]
INSTEAD OF INSERT
AS
DECLARE #unitprice float = (SELECT TOP 1 unitprice from inserted )
DECLARE #amount float = (SELECT TOP 1 amount from inserted)
INSERT INTO HR.[Stuff]
VALUES(#unitprice,#amount, #unitprice*#amount)
GO
CREATE TRIGGER [calculate_price_update]
ON [HR].[Stuff]
INSTEAD OF UPDATE
AS
DECLARE #unitprice float = (SELECT TOP 1 unitprice from inserted )
DECLARE #amount float = (SELECT TOP 1 amount from inserted)
UPDATE HR.[Stuff]
SET unitprice = #unitprice, amount = #amount, price = #unitprice*#amount
WHERE unitprice = (SELECT TOP 1 saleid from inserted)
GO

Answer 1

You need to use computed column:
CREATE TABLE [HR].[Stuff](
[saleid] [int] IDENTITY(1,1) NOT NULL,
[unitprice] [float] NOT NULL,
[amount] [float] NOT NULL,
[price] AS ([unitprice] * [amount])
)
LiveDemo
Also storing unitprice and amount as FLOAT may be dangerous because FLOAT isn't accurate datatype. Use DECIMAL(12,4) instead.
Your original trigger solution will fail, because trigger is executed per statement, not per row. Try with:
INSERT INTO [HR].[Stuff](unitprice, amount) VALUES (10, 20), (30, 50), (100,1);
You will loose records with INSERT and get false results with multiple UPDATE.
SqlFiddleDemo