> Wouldn't VAL_A be included in SUM(VAL_A), so other records must be 0

or negative to not break this rule? Or do you mean SUM(Link.VAL_A) must
be less than A.VALUE?

Yes, I meant the sum of Link.VAL_A, not the parent table.

> You could also perform a dummy update like
> update a set value = value where id = ?;
> Or use a select with lock.

I just looked up 'select...with lock' in The Firebird Book, and it warns
that it can't be used with joins or aggregations.
Six pages earlier it mentions that Firebird locks by row, not column.
So my idea of a timestamp on the parent table would mean that any
updates to the parent's row would be blocked until the transaction ends.
Which is good, as there are other tables that are all linked to table A,
and the summation of all these links can't exceed the parent's VALUE.

Thanks, I've got more to think about before I implement, but I think the
concept is now sound.

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