If you need to add a new column in TSQL. You need to use the keyword function 'ALTER TABLE'. It is recommended you perform a check if the proposed column you want to add is not already existed in the database table otherwise you will receive an error message.

Let says we have a table called products with the following columns of information.

If we want to add a column named 'Description' with column type NVARCHAR(MAX) and we want to set the default value for a new column with an empty string and do not allow NULL value.

Here is the full TSQL query to add a new column in SQL Server.

IF NOT EXISTS(SELECT 1 FROM sys.columns
          WHERE Name = N'Description'
          AND Object_ID = Object_ID(N'dbo.Products'))
BEGIN
    ALTER TABLE dbo.Products
    ADD Description NVARCHAR(MAX) DEFAULT '' NOT NULL
END

GO

Here is the final design of the table after we add the new column.