SET NOCOUNT ON

DECLARE  @constraintname SYSNAME,@objectid int,@sqlcmd         VARCHAR(1024)

DECLARE CONSTRAINTSCURSOR CURSOR  FOR
SELECT NAME,object_id
FROM   SYS.OBJECTS
WHERE  TYPE = 'D' AND @objectid = OBJECT_ID('Mytable')

OPEN CONSTRAINTSCURSOR

FETCH NEXT FROM CONSTRAINTSCURSOR
INTO @constraintname,@objectid

WHILE (@@FETCH_STATUS = 0)
BEGIN
    SELECT @sqlcmd = 'ALTER TABLE ' + OBJECT_NAME(@objectid) + ' DROP CONSTRAINT ' + @constraintname
    EXEC( @sqlcmd)
    FETCH NEXT FROM CONSTRAINTSCURSOR
    INTO @constraintname,@objectid
END

CLOSE CONSTRAINTSCURSOR
DEALLOCATE CONSTRAINTSCURSOR