updating table field whenever other table field changes

Hello postgresql.questions group!

What do you think the best way to attack the following is:
parent table a - [status_field]

child table b - [status_field]

I need trigger or similar to achieve the following:
if a.status_field changes than all children of table a have their
b.status_field change to the same value as a.

However if b.status_field changes its parent record, a.status_field,
does not change.

Should I just create a separate update trigger or can I set this up
right when I am declaring these tables to achive this.

I also do not really understand CONSTRAINT TRIGGER and how it differs
from a regular old TRIGGER. What do these internal TRIGGERS,
particularly "RI_FKey_noaction_upd" do?

Thanks for your time. Any help or advice is appreciated.

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Andrew C.
tech7890(at)NOTPART@yahoo.com
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me