calculating average of star rating system using .net and sql server 2005

We are facing problem in calculating average of votes or stars for a particular restaurant name or id

We have two tables named Restaurant and Comments Restaurants has foolowing columns RestId,RestName,Rating,.......... Comments has RestId,RestName,Rating,Average.........

In comments table we store comments with rating(1,2,3,4,5) in Rating column and Average is to be calculated and stored in Average column This Average column value is then updated in Reataurant table in Rating column Actually we are trying to calculate average using "select avg(rating) from comments where RestName=''" and directly storing value into Restaurant table without using average column

set ANSI_NULLS ON                    
set QUOTED_IDENTIFIER ON                    
GO                    
ALTER procedure [dbo].[average](@RestId varchar(50))                    
as                    
update restaurants set rating=(select avg(rating) from comments where RestName=@RestId)                    
                    
exec average '1'                    
                    
select * from comments                    
                    
select * from restaurants                    
                    
select avg(rating) from comments where RestName='Mcdonald'                    

then also tried

create proc rate_calculate(@RestName varchar(50))                    
as                    
begin                    
select res.RestName,res.rating                    
from restaurants as res                    
left join(select RestName,avg(rating) as average from Comments where RestName=@RestName) as c on c.RestName=res.RestName                    
go                    

but still we are not getting the average per restaurant

We want a procedure to calculate average of rating from comments table and storing the value into restaurant table

EDIT -- Additional information

The result which i got is:- In the comments table individual rating is stored but taking avg(rating) from comments table it has to store the value into restaurants table for each restaurant but same avg(rating) is stored in each restaurant

> select * from comments                    
                    
    42 excellent1 sajeed@gmail.com test NULL 2010-02-16 13:50:14.000 5 Mcdonald                    
----------                    
    43 excellent 2 suja.p03@gmail test NULL 2010-02-16 14:31:54.000 5 Pizza Hut                    
----------                    
    44 excellent 1 sajeed@gmail.com test	NULL	2010-02-16 14:51:19.000	5	Mcdonald                    
----------                    
    45  good	1	sajeed@gmail.com	test	NULL	2010-02-16 15:08:32.000	3	Mcdonald                    
----------                    
    46  good	1	sajeed@gmail.com	test	NULL	2010-02-16 15:16:18.000	1	Mcdonald                    
----------                    
    47  good	2	sajeed@gmail.com	test	NULL	2010-02-16 15:22:08.000	5	Pizza Hut**                    
----------                    
                    
                    
> select avg(rating) as average from comments where RestName='Mcdonald'                    
                    
average                    
3.500000                    
                    
                    
> select avg(rating) as average from comments where RestName='Pizza Hut'                    
                    
average                    
5.000000                    

This is the result but its taking round() 3.50000 but it has to be 3.5000 and 5.0000

> select * from restaurants                    
RestName   Rating                    
Mcdonald   4                    
Pizza Hut  4                    

so please help