I have the following table structure in my Postgres DB (v12.0)
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}
I am using GROUP BY query for this records, like below
SELECT SUM(PIECES) FROM detail_table GROUP BY item_id HAVING item_id =2
Using which I will get the total pieces as 30. But how could I get the count of total pieces from material_detail group by material_id.
I want result something like this
pieces | material_detail
-------| ------------------
30 | [{"material_id":1,"pieces":50},{"material_id":2,"pieces":20},{"material_id":3,"pieces":30}]
As I am from MySQL background, I don’t know how to achieve this with JSON fields in Postgres.
Note: material_detail column is of JSONB type.
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Answer
You are aggregating on two different levels. I can’t think of a solution that wouldn’t need two separate aggregation steps. Additionally to aggregate the material information all arrays of the item_id have to be unnested first, before the actual pieces value can be aggregated for each material_id. Then this has to be aggregated back into a JSON array.
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
;