Drop and recreate primary key on a table

-- declare a variable to hold the name of the primary key
DECLARE @sPrimaryKeyName varchar(300)
-- get the name of the primary key, if any
SELECT @sPrimaryKeyName = a.[name]
from sys.indexes a
where a.[object_id] = object_id('MyTable')
    and a.is_primary_key = 1

-- see if we got back a primary key name
if (@sPrimaryKeyName is not null)
begin
    -- we have a primary key on the table, drop it now.
    exec ('ALTER TABLE [MyTable] DROP CONSTRAINT ' + @sPrimaryKeyName)
end
GO

--use this line if the primary key is to have a specific name:
--ALTER TABLE [dbo].[MyTable] ADD CONSTRAINT [PK_MyTable] PRIMARY KEY CLUSTERED 
-- add the primary key back on the table
ALTER TABLE [dbo].[MyTable] ADD PRIMARY KEY CLUSTERED 
(
    [Col1] ASC,
    [Col2] ASC,
    [Col3] ASC
) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) 
ON [PRIMARY]
GO