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

Try this:

ALTER TABLE tablename ADD newcolumn type NOT NULL DEFAULT (0)
Go
Update tablename SET newcolumn = oldcolumn Where newcolumn = 0
Go

Answer 2

To extend Kapil's answer, and avoid the unwanted default constraint, try this:

ALTER TABLE tablename ADD newcolumn type NOT NULL CONSTRAINT DF_TMP_TABLENAME_NEWCOLUMN DEFAULT -9999
Go
Update tablename SET newcolumn = oldcolumn
Go
ALTER TABLE tablename DROP CONSTRAINT DF_TMP_TABLENAME_NEWCOLUMN
Go

Replace -9999 by 'noData' if your type is varchar, nvarchar, datetime,... or by any compatible data for other types: specific value doesn't matter, it will be wiped by the 2nd instruction.

Answer 3

I don't like them very much but here is how you could do this with an AFTER INSERT trigger:

CREATE TRIGGER TableX_AfterInsert_TRG 
  ON TableX 
AFTER INSERT
AS
  UPDATE TableX AS t
  SET t.newcolumn = t.oldcolumn
  FROM Inserted AS i
  WHERE t.PK = i.PK ;              -- where PK is the PRIMARY KEY of the table   

Answer 4

For my case, I want to add a new not null unique column named CODE but I don't know about the value at the creation time. I set the default value for it by get a default value from NewID() then update later.

ALTER TABLE [WIDGET] ADD [CODE] CHAR(5) NOT NULL DEFAULT(SUBSTRING(CONVERT(CHAR(36), NEWID()), 1, 5))

ALTER TABLE [dbo].[WIDGET] WITH CHECK ADD CONSTRAINT [UQ_WIDGET_CODE] UNIQUE ([CODE])

Answer 5

I think it will work if you use Set Identity_Insert <TableName> OFF and after the insert Statement that you wrote just use Set Identity_Insert <TableName> ON.

Answer 6

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.