Add a column to a table with a default value equal to the value of an existing column

Question

How to add a column to a SQL Server table with a default value that is equal to value of an existing column?

I tried this T-SQL statement:

ALTER TABL         


        

Answer 1

The AFTER INSERT trigger approach involves overhead due to the extra UPDATE statement. I suggest using an INSTEAD OF INSERT trigger, as follows:

CREATE TRIGGER tablename_on_insert ON tablename 
INSTEAD OF INSERT 
AS
INSERT INTO tablename (oldcolumn, newcolumn)
SELECT oldcolumn, ISNULL(newcolumn, oldcolumn)
FROM inserted

This does not work though if the oldcolumn is an auto-identity column.