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

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