help in SP

Here is my SP, i m using SQL Server 2000, in the Severity Column, i have 3 values : Critical, Information and Error, But when i
run my SP : Exec GetFilteredLog null,'Critical' , it gives the following msg:


Invalid column name 'Critical'.When i check the Query it makes this:
Select LogID,Severity,UserName,Timestamp,MachineName,PayLoad,Message,Priority,CategoryID
from Log Where Severity in (Critical), if u checked here it not add the '' in the Value CRITICAL, i tried to concatenate
with Comma, but stll gives error , kindly tell me how can i get this as this is varchar value.



CREATE PROCEDURE GetFilteredLog

@CategoryID varchar(50) = null ,
@Severity varchar(50) = null ,
@UserActivityCode varchar(50)= null ,
@Priority varchar(50) = null
AS

Declare @Select nvarchar(4000)
Declare @From nvarchar(4000)
Declare @Where nvarchar(4000)
Declare @SQL nvarchar(4000)


select @Select = ' Select LogID,Severity,UserName,Timestamp,MachineName,PayLoad,Message,Priority,CategoryID'

select @From = 'from Log'

IF @CategoryID is not null

BEGIN
Select @Where = 'Where CategoryID in ('+ @CategoryID+ ')'

END

IF @Severity is not null
BEGIN

Select @Where = Case When @Where is null then 'Where Severity in ('+ @Severity+')'
Else @Where + ' And '+ 'Severity in ('+ '@Severity'+ ')'
End

END

IF @UserActivityCode is not null
BEGIN
Select @Where = Case When @Where is null then 'Where UserActivityCode in ('+ @UserActivityCode+ ')'
Else @Where + ' AND '+ 'UserActivityCode in ('+ @UserActivityCode+ ')'
End

END

IF (@Priority is not null)
BEGIN
Select @Where = Case When @Where is null then 'Where Priority in ('+ @Priority+ ')'
Else @Where + ' AND '+ 'Priority in ('+ @Priority+ ')'
End

END

Set @SQL = @Select +' '+ @From +' '+ Coalesce(@Where,'')
select @SQL
Exec sp_executesql @SQL

Kindly reply me, Thanx in Advance.

Thankyou so much, its working now, i missed one quote at the end of @Serverity. Once again thanx.