SQL Server - SQL Table Basics - Altering/Adding Columns

Adding column

You can add new columns to an existing table. .

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

GO

You can add modify an existing column

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

GO

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.
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.

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