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

Default value: Explanation and integrity

Setting a default value for a new column implies that, at the very moment of insertion, if no value is assigned for this column, SQL unhesitatingly steps in, filling it up with the default you specified. No-nonsense business here!

Nevertheless, SQL gets a bit "laissez-faire" for existing records. It shrugs off and won't fill the new column with the default value. For that, you must explicitly command it.

Alternatives dancing: Computed columns

Availability is key in database relationships. If you want the new column to always have the existing column's data, don't knock twice! Welcome a computed column into your life:

ALTER TABLE table_name
ADD new_col AS (existing_col) PERSISTED;


A computed column uses a taped formula to automatically assign its value. Using the PERSISTED keyword is like buying it a storage unit, enhancing performance since it doesn't need recalculation with each query. It's the gift that keeps on giving!

Triggers: the data bouncer

Another alternative road is through INSTEAD OF INSERT triggers. It's like having a bouncer at the entrance of your data club, making sure new rows abide by the code:

CREATE TRIGGER trg_tableName_initNewCol
ON table_name
INSTEAD OF INSERT
AS
BEGIN
    INSERT INTO table_name (column1, column2, new_col)
    SELECT column1, column2, existing_col
    FROM inserted;
END;


Null values: never forget!

In the SQL jungle, NULL values are sneaky creatures. However, armed with the ISNULL function, you'll handle potential nulls like a pro:

UPDATE table_name
SET new_col = ISNULL(existing_col, 'DefaultValue');


System impact: Proceed with caution

Just like cooking a soufflé, timing is everything. Introducing changes to your database requires careful consideration.

Wave "hello" to your applications

Broaden the puzzle. Examine your application code, particularly if it swings hands with the database schema.

Performance: what's the cost?

Think twice before employing triggers or computed columns – they might leave footprints on your performance. Bring out your detective gear and thoroughly measure your query performance pre and post these changes.

Database expert: no shame in asking for help

Stuck in a complex scenario with data as vast as the sea and intricate relationships? Time to Yelp a database expert to navigate these waters without capsizing your boat.