Question

I want to alter a table and add a column that is the sum of two other columns, and this column is auto computed when I add new data.

Answer 1

Rather than adding this column to the table, I would recommend using a View to calculate the extra column, and read from that.

Here is a tutorial on how to create views here:

http://odetocode.com/Articles/299.aspx

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Your view query would look something like:

 SELECT 
    ColumnA, ColumnB, (ColumnA+ColumnB) as ColumnC
  FROM 
    [TableName]