fieldB = fieldA
if fieldA is changing, fieldB not changing with it. Namely fieldB = the first value entry to fieldA.
What would be the purpose of that?
Is fieldB STORED?
Knowing what those fields represent using real table/field names helps us visualize the type of functionality you require. Without understanding this perspective and context and the purpose of the request, we are throwing darts blindfolded. :-)
Retrieving data ...