在 postgres 中使用 GROUP BY 子句時如何聚合 json 字段？

Question

我的 Postgres DB (v12.0) 中有以下表結構

id | pieces | item_id | material_detail
---|--------|---------|-----------------
1  | 10     | 2       | [{"material_id":1,"pieces":10},{"material_id":2,"pieces":20},{"material_id":3,"pieces":30}]

2  | 20     | 2       | [{"material_id":1,"pieces":40}
3  | 30     | 3       | [{"material_id":1,"pieces":20},{"material_id":3,"pieces":30}

我正在對這些記錄使用 GROUP BY 查詢，如下所示

SELECT SUM(PIECES) FROM detail_table GROUP BY item_id HAVING item_id =2 

使用它我將獲得總件數為 30。但是我如何通過 material_id 從 material_detail 組中獲得總件數。

我想要這樣的結果

 pieces |  material_detail
 -------| ------------------
  30    |  [{"material_id":1,"pieces":50},{"material_id":2,"pieces":20},{"material_id":3,"pieces":30}]

由於我來自 MySQL 背景，我不知道如何使用 Postgres 中的 JSON 字段來實現這一點。

注意： material_detail 列是 JSONB 類型。

Answer 1

您在兩個不同的級別上進行聚合。 我想不出不需要兩個單獨的聚合步驟的解決方案。 此外，要聚合材料信息，必須首先取消 item_id 的所有 arrays，然后才能為每個 material_id 聚合實際件值。 然后必須將其聚合回 JSON 數組。

with pieces as (

  -- the basic aggregation for the "detail pieces"
  select dt.item_id, sum(dt.pieces) as pieces
  from detail_table dt
  where dt.item_id = 2
  group by dt.item_id

), details as (

  -- normalize the material information and aggregate the pieces per material_id
  select dt.item_id, (m.detail -> 'material_id')::int as material_id, sum((m.detail -> 'pieces')::int) as pieces
  from detail_table dt
    cross join lateral jsonb_array_elements(dt.material_detail) as m(detail)
  where dt.item_id in (select item_id from pieces) --<< don't aggregate too much    
  group by dt.item_id, material_id

), material as (

  -- now de-normalize the material aggregation back into a single JSON array
  -- for each item_id
  select item_id, jsonb_agg(to_jsonb(d) - 'item_id') as material_detail
  from details d
  group by item_id

)
-- join both results together
select p.item_id, p.pieces, m.material_detail
from pieces p
  join material m on m.item_id = p.item_id
; 

在線示例