title: Replace Null columns in ssis
body: <p>I want to add a column named <code>CategoryCode</code> which would take the first 3 letters of <code>Category</code> column + the first 3 letters of <code>Subcategory</code> column+ any number.
  The problem is that I have some values in <code>CategoryCode</code> column which I don't have to change. I just have to operate on values which is null.
  Can anyone help me with that? It would be of great help.</p>