Given a table like this:
create table primary ( a integer primary key, b boolean
);
And another like this:
create table secondary ( a integer, some_other_fields
);
I would like a foreign key constraint on the "secondary" table that
looks something like:
foreign key (a, true) references primary (a, b)
That is, a row in "secondary" is allowed to reference a row in "primary"
if and only if that referenced row has (b = true).
I think I have to do this with an additional trigger. Any other options?