使用 PostgreSQL 动态查询 JSON

Question

I have a json value like the below which I am wanting to extract details from in a PostgreSQL Database.

[{"monday":{"opens":730,"closes":2300}},{"tuesday":{"opens":730,"closes":2300}},{"wednesday":{"opens":730,"closes":2300}},{"thursday":{"opens":730,"closes":2300}},{"friday":{"opens":730,"closes":2300}},{"saturday":{"opens":730,"closes":2300}},{"sunday":{"opens":730,"closes":2300}}]

Currently I am using the following syntax to extract opening hours which returns '730' in this instance

s.opening_hours -> 0 -> 'monday' ->> 'opens' 

However I want it to be more dynamic by being able to obtain opening hours without specifying 'Monday' as sometimes the value does not start with Monday, eg sometimes 'Tuesday'.

Attempts so far to replace 'Monday' with the number position have return 'null' eg

s.opening_hours -> 0 -> 0 ->> 'opens' , -- null

Answer 1

Well, you could do a lateral join with a derived table getting the first key of the object in question using jsonb_object_keys() WITH ORDINALITY ans use that key in your expression.

SELECT t.opening_hours->0->x.k->'opens'
       FROM (VALUES ('[{"monday":{"opens":730,"closes":2300}},{"tuesday":{"opens":730,"closes":2300}},{"wednesday":{"opens":730,"closes":2300}},{"thursday":{"opens":730,"closes":2300}},{"friday":{"opens":730,"closes":2300}},{"saturday":{"opens":730,"closes":2300}},{"sunday":{"opens":730,"closes":2300}}]'::jsonb)) AS t
                                                                                                                                                                                                                                                                                                                              (opening_hours)
            CROSS JOIN LATERAL (SELECT jok.k
                                       FROM jsonb_object_keys(t.opening_hours->0) WITH ORDINALITY jok
                                                                                                  (k,
                                                                                                   o)
                                       ORDER BY jok.o
                                       LIMIT 1) x;

db<>fiddle