Answer by gdmanandamohon for Add a column with a default value to an existing table in SQL Server

Well, I now have some modification to my previous answer. I have noticed that none of the answers mentioned IF NOT EXISTS. So I am going to provide a new solution of it as I have faced some problems altering the table.

IF NOT EXISTS (SELECT * FROM INFORMATION_SCHEMA.columns WHERE table_name = 'TaskSheet' AND column_name = 'IsBilledToClient')
BEGIN
ALTER TABLE dbo.TaskSheet ADD
 IsBilledToClient bit NOT NULL DEFAULT ((1))
END
GO

Here TaskSheet is the particular table name and IsBilledToClient is the new column which you are going to insert and 1 the default value. That means in the new column what will be the value of the existing rows, therefore one will be set automatically there. However, you can change as you wish with the respect of the column type like I have used BIT, so I put in default value 1.

I suggest the above system, because I have faced a problem. So what is the problem? The problem is, if the IsBilledToClient column does exists in the table table then if you execute only the portion of the code given below you will see an error in the SQL server Query builder. But if it does not exist then for the first time there will be no error when executing.

ALTER TABLE {TABLENAME}
ADD {COLUMNNAME} {TYPE} {NULL|NOT NULL}
CONSTRAINT {CONSTRAINT_NAME} DEFAULT {DEFAULT_VALUE}
[WITH VALUES]