我没有像@MikeOrganek那样为自连接生成行id，而是只需要unnest两次就可以生成一个具有重复元组的关系：

SELECT x AS item, json_object_agg(y, count ORDER BY count DESC) AS co_occurences
FROM (
  SELECT x, y, count(*)
  FROM example, unnest(items) AS x, unnest(items) AS y
  WHERE x <> y
  GROUP BY x, y
) tmp
GROUP BY x;

字符串
（online demo）

首先将其转换为正常的关系形式。
以下代码为任意行id赋值：

with create_ids as (
  select row_number() over (order by items) as id,
         items
    from item_groups
), normalize as (
  select i.id, u.item
    from create_ids i
         cross join lateral unnest(i.items) as u(item)
), correlate as (
  select a.item, b.item as coitem, count(b.item) as occurence_count
    from normalize a
         left join normalize b
           on b.id = a.id and b.item != a.item
   group by a.item, b.item
)fiddle
select item, jsonb_object_agg(coitem, occurence_count) as co_occurences
  from correlate 
 group by item;

字符串
工作fiddle
如果目标不一定是JSON，那么我们可以构造一个字符串。这允许对字符串中的值进行排序：

with create_ids as (
  select row_number() over (order by items) as id,
         items
    from item_groups
), normalize as (
  select i.id, u.item
    from create_ids i
         cross join lateral unnest(i.items) as u(item)
), correlate as (
  select a.item, b.item as coitem, count(b.item) as occurence_count
    from normalize a
         left join normalize b
           on b.id = a.id and b.item != a.item
   group by a.item, b.item
)
select item, array_agg(coitem||':'||occurence_count order by occurence_count desc) as co_occurences
  from correlate 
 group by item;

型
更新fiddle