I have a hierarchical structure in a database driven software application.

Each row in parentTable 'owns' a number of rows in childTable.

If the childTable does not have a value set in columnA then it should return the value specified in the appropriate row of parentTable columnB.

Is it possible to implement a pointer or cell reference somehow so that I do not have to copy values from parent to child.

(A change in parent could result in a large number of changes in child).

If this is not possible, could anyone suggest a different approach?