Sum of two calculated columns in SQL
<p>I'm calculating two rows like this : </p>

<pre><code>CASE 
    WHEN isnumeric(SUBSTRING(dbo.Table.RNumber,1,CHARINDEX('+',dbo.Table.RNumber) - 1)) = 1
        THEN SUBSTRING(dbo.Table.RNumber,1,CHARINDEX('+',dbo.Table.RNumber) - 1) 
    else 0 
end AS RoomNumber,
CASE 
    WHEN isnumeric(SUBSTRING(dbo.Table.R.Number,CHARINDEX('+',dbo.Table.RNumber) + 1,LEN(dbo.Table.R.Number))) = 1
        THEN SUBSTRING(dbo.Table.R.Number,CHARINDEX('+',dbo.Table.RNumber) + 1,LEN(dbo.Table.R.Number)) 
    else 0 
end AS HallNumber,
</code></pre>

<p>I need to make another column that calculates the sum of RoomNumber and HallNumber, I tried this :</p>

<pre><code>SELECT *, (RoomNumber + HallNumber) AS Sum
FROM Table
</code></pre>

<p>But I got invalid column name error.
How can I get the sum of these two columns? Thanks.</p>