title: Alter table to modify default value of column
body: <p>I have a requirement where we need to modify a column's default value in database table. The table is already an existing table in database and currently the default value of the column is NULL.
  Now if add a new default value to this column, If I am correct it updates all the existing NULLs of the column to new DEfault value. Is there a way to not to do this but still set a new default value on column.
  I mean I do not want the existing NULLs to be updated and want them to remain as NULLs.</p>
  
  <p>Any help on this is appreciated.
  Thanks</p>