SQL question: checking all required items

Hi all,

Given the following tables -

create table people (
person_id text primary key,
person_name text,
[...etc...]
);

create table items (
item_id text primary key,
item_name text,
is_required boolean,
[...etc...]
);

create table items_for_people (
person_id text,
item_id text,
primary key (person_id, item_id),
foreign key person_id references people(person_id),
foreign key item_id references items(item_id)
);

- how can I find those people who don't have _all_ of the items which
are marked "required"?

In other words, how do I select those rows in "people" which don't have
a corresponding row in "items_for_people" for *each* row in "items"
which has is_required=true?

Many thanks,

Ray.

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Raymond O'Donnell, Director of Music, Galway Cathedral, Ireland
rod(at)iol(dot)ie
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