Looping JSON Array in JSONB field - sql

Question

I want to loop over JSONB column and get certain values (price, discount_price, and currency) of relevant JSON objects to my filter. But I get this error:
syntax error at or near "FOR"
Value of the parts column which is JSONB:
[
{
"item_tags": ["black", "optional"],
"name": "Keyboard",
"price": 50,
"currency": "USD",
"discount_price": 40
},
{
"item_tags": ["white", "optional"],
"name": "Mouse",
"price": 40,
"currency": "USD",
"discount_price": 30
}
]
My query ($1 is the user input. Can be 'optional' or 'required'):
SELECT
id,
title,
FOR element IN SELECT * FROM jsonb_array_elements(parts)
LOOP
CASE
WHEN element->'item_tags' #> $1
THEN SELECT element->>'discount_price' AS price, element->>'currency' AS currency
ELSE SELECT element->>'price' AS price, element->>'currency' AS currency
END
END LOOP
FROM items;
This is the output I want to get if $1 is equal to 'optional':
{
"id": 1,
"title": "example title",
"parts": [
{
"name": "Keyboard",
"discount_price": 40,
"currency": "USD"
},
{
"name": "Mouse",
"discount_price": 30,
"currency": "USD"
}
]
}
Any help is highly appreciated. I follow official docs but it is not beginner-friendly. I use PostgreSQL 13.

Answer 1

You need to unnest the array, filter out the unwanted parts, remove the unwanted key, then aggregate the changed parts back into a JSON array.
This can be done using a scalar sub-query:
select id, title,
(select jsonb_agg(x.part - 'item_tags')
from jsonb_array_elements(i.parts) as x(part)
where (x.part -> 'item_tags') ? 'optional')
from items i;
The expression x.part - 'item_tags' removes the item_tags key from the JSON object. The ? operator tests if the item_tags array contains the string on the right hand side. And jsonb_agg() then aggregates those JSON values back into an array.
You can pass your parameter in the place of the 'optional' string.