Alter Table ,add Column ,alter Column datatype

Adding column

You can add new columns to an existing table. 

ALTER TABLE [dbo].[TableName]
ADD   NameOfNewColumn Varchar(500) NULL  

GO

Alter column

You can add modify an existing column

ALTER TABLE [dbo].[TableName]
 ALTER COLUMN [NameOfNewColumn ]  int NULL

GO

Drop column

You can add modify an existing column

ALTER TABLE [dbo].[TableName]
Drop [NameOfNewColumn ]  int

GO 

For more details visit msdn link

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