Postgresql 如何对行进行分组并生成另一列

我有下面的示例表。 一个组id正好有两个项，一个主项和一个父项

SELECT
  'id1' as id,
  'parent' as mode,
  '1' as group_id,
   10 as value
UNION ALL
SELECT
  'id2' as id,
  'parent' as mode,
  '2' as group_id,
   11 as value
UNION ALL
SELECT
  'id3' as id,
  'main' as mode,
  '1' as group_id,
   20 as value
UNION ALL
SELECT
  'id4' as id,
  'main' as mode,
  '2' as group_id,
   22 as value

我想要输出

 parent_value | parent_id | main_value | main_id | group_id
     10            id1          20         id3         1
     11            id2          22         id4         2

我尝试过用以下方式思考，但无法成功地聚合

SELECT
 -- I need parent value
 -- I need main value
 -- I need main id,
 group_id
FROM (
...
) as a1 GROUP BY group_id