SQL question: checking all required items

["Raymond O'Donnell" <rod@xxxxxx>]

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.

---------------------------------------------------------------
Raymond O'Donnell, Director of Music, Galway Cathedral, Ireland
rod@xxxxxx
---------------------------------------------------------------

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TIP 9: In versions below 8.0, the planner will ignore your desire to
      choose an index scan if your joining column's datatypes do not
      match