Can get an average of values in a json array using postgres?

问题

One of the great things about postgres is that it allows indexing into a json object.

I have a column of data formatted a little bit like this:

{"Items":
  [
    {"RetailPrice":6.1,"EffectivePrice":0,"Multiplier":1,"ItemId":"53636"},
    {"RetailPrice":0.47,"EffectivePrice":0,"Multiplier":1,"ItemId":"53404"}
  ]
}

What I'd like to do is find the average RetailPrice of each row with these data.

Something like

select avg(json_extract_path_text(item_json, 'RetailPrice')) 

but really I need to do this for each item in the items json object. So for this example, the value in the queried cell would be 3.285

How can I do this?

回答1:

Could work like this:

WITH cte(tbl_id, json_items) AS ( 
   SELECT 1
        , '{"Items": [
       {"RetailPrice":6.1,"EffectivePrice":0,"Multiplier":1,"ItemId":"53636"}
      ,{"RetailPrice":0.47,"EffectivePrice":0,"Multiplier":1,"ItemId":"53404"}]}'::json
   )
SELECT tbl_id, round(avg((elem->>'RetailPrice')::numeric), 3) AS avg_retail_price
FROM   cte c
     , json_array_elements(c.json_items->'Items') elem
GROUP  BY 1;

The CTE just substitutes for a table like:

CREATE TABLE tbl (
   tbl_id     serial PRIMARY KEY
 , json_items json
);

• json_array_elements() (Postgres 9.3+) to unnest the json array is instrumental.

• I am using an implicit JOIN LATERAL here. Much like in this related example:

  • Query for element of array in JSON column

• For an index to support this kind of query consider this related answer:

  • Index for finding an element in a JSON array

• For details on how to best store EAV data:

  • Is there a name for this database structure?

来源：https://stackoverflow.com/questions/25512616/can-get-an-average-of-values-in-a-json-array-using-postgres