PostgreSql : Json Array to Rows using Lateral Join

Question

I have two following JSON Array in details field of my table and need to evaluate the query as I use in another relational table.

{
    \"city\": \"London\",         


        

Answer 1

select
  quantity, price,
  avg(quantity/price) as ratio
from my_table cross join lateral (
  select
    json_array_elements_text(details->'dates') as dates,
    (json_array_elements_text(details->'quantities'))::numeric as quantity,
    (json_array_elements_text(details->'prices'))::numeric as price) as data
where details->>'city' = 'London'
group by quantity, price;

Answer 2

Is this what you want ?

    -- just simulate table:
with my_table(details) as(
values
('{
"city": "London",
"name": "Sainburry",
"quantities": [112, 145, 222, 122, 124],
"prices": [4, 4, 4, 0, 3],
"dates": ["13.05.2020", "14.05.2020", "15.05.2020", "16.05.2020", "17.05.2020"]
}'::json)
)


-- here is query:
select  
my_table.details->>'city',  u.quantities, u.prices  
from my_table
JOIN LATERAL UNNEST( 
    ARRAY(SELECT json_array_elements_text(details->'quantities')) ,
    ARRAY(SELECT json_array_elements_text(details->'prices')) 
) u(quantities, prices) ON TRUE
WHERE
my_table.details->>'city' = 'London'

See demo

Answer 3

Seems you need WITH ORDINALITY along with LEFT JOIN LATERALs to match the corresponding elements of the arrays due to the order in the arrays, respectively :

SELECT q.elm AS quantities, p.elm AS prices, 
       AVG(p.elm::float/q.elm::float) AS ratio
  FROM my_table t0
  LEFT JOIN LATERAL jsonb_array_elements(details -> 'quantities') 
    WITH ORDINALITY AS q(elm, i) ON TRUE
  LEFT JOIN LATERAL jsonb_array_elements(details -> 'prices') 
    WITH ORDINALITY AS p(elm, i) ON q.i = p.i
  LEFT JOIN LATERAL jsonb_array_elements(details -> 'dates') 
    WITH ORDINALITY AS d(elm, i) ON d.i = q.i
 WHERE t0.details ->> 'city' = 'London'   
 GROUP BY q.elm, p.elm;

Demo

Answer 4

Basically, what Abelisto said. Just a couple of improvements and some explanation:

SELECT to_date(d, 'DD.MM.YYYY') AS date -- ①
     , quantity, price
     , round(price / quantity, 4) AS ratio -- ③, ④
FROM   my_table
CROSS  JOIN LATERAL ( -- ②
   SELECT json_array_elements_text(details->'dates'     )          AS d -- ①
        , json_array_elements_text(details->'quantities')::int     AS quantity -- ③
        , json_array_elements_text(details->'prices'    )::numeric AS price -- ③
   ) AS data
WHERE  details->>'city' = 'London';

db<>fiddle here

① Date strings are interpreted depending on locale settings and session variables by default. Do it the clean way with to_date().

② Multiple set-returning functions in the SELECT list behave in surprising ways up until Postgres 10 if the number of resulting rows is not exactly the same for all. (Consider upgrading. In any case.) See:

• What is the expected behaviour for multiple set-returning functions in SELECT clause?

③ In your original query AVG(quantities/prices::float) makes no sense in combination with group by quantities, prices. Neither does quantities/prices on its own. I fixed as I saw fit, and threw in round() to format output.

④ If quantity can be 0 defend against division by 0 with NULLIF:

     , round(price / NULLIF(quantity, 0), 4) AS ratio