SQL Server - Altering / Adding Columns

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

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ALTER TABLE mytable
ALTER COLUMN mycolumn datatype(new_width)