ALTER TABLE

Add New Column:

ALTER TABLE [dbo].[SampleTable] ADD [NewColum_1] NVARCHAR(MAX) -- will be added as NULLable
ALTER TABLE [dbo].[SampleTable] ADD [NewColum_1] NVARCHAR(MAX) NULL
ALTER TABLE [dbo].[SampleTable] ADD [NewColum_1] NVARCHAR(MAX) NOT NULL

Add Primary Key:

ALTER TABLE [dbo].[SampleTable] ADD CONSTRAINT PK_Sample_Table PRIMARY KEY CLUSTERED (Id)

Change data type for Primary Key column (IDENTITY setting will be persisted):

-- drop primary key constraint
ALTER TABLE [dbo].[SampleTable] DROP CONSTRAINT [PK_SampleTable]
GO

-- alter column
ALTER TABLE  [dbo].[SampleTable] ALTER COLUMN [Id] INT NOT NULL -- e.g. changed from SMALLINT to INT

-- recreate primary key constraint
ALTER TABLE [dbo].[SampleTable] ADD CONSTRAINT [PK_SampleTable] PRIMARY KEY CLUSTERED ( [Id] ASC )
GO