Add Column

Adding column
You can add new columns to an existing table. .

ALTER TABLE [dbo].[phone]
ADD inactive_date DATETIME NULL
GO

Alter column
You can add modify an existing column

ALTER TABLE [dbo].[person]
ALTER COLUMN [lastname] VARCHAR(35) NULL
GO

Considerations for altering a column
1. Reducing precision (example, going from CHAR(20) to CHAR(15)) can cause data truncation and should be avoided unless you are absolutely sure their will be no impact to the data.
2. Changing data types should typically be avoided. There are exceptions to this. For example, changing a CHAR(20) to a VARCHAR(20) on columns where the average storage length is 10 can save disk space.

Alter columns - No Can Do
You cannot directly alter a a column that is part of the primary key