How to count instances inside json object array? - sql

Question

I have some table that includes an array of jsonb objects as a column:
| event_id | attendees |
|----------|----------------------------------------------------------------------------------------------------------------------------------------------|
| 1 | [{"name": "john smith", "username": "jsmith"}, {"name": "jeff jones", "username": "jjones"}, {"name": "steve woods", "username": "swoods"}] |
| 2 | [{"name": "al williams", "username": "awilliams"}, {"name": "james lee", "username": "jlee"}, {"name": "bob thomas", "username": "bthomas"}] |
| 3 | [{"name": "doug hanes", "username": "dhanes"}, {"name": "stan peters", "username": "speters"}, {"name": "jane kay", "username": "jkay"}] |
I would like to get the count of all attendees whose username matches some condition (let's say whose username starts with "j") for each event.
Looking at the documentation, I couldn't really find anything that I could use for jsonb object arrays. The closest thing I could see was the jsonb_array_elements function, but that returns a set and not individual values. so something like:
select event_id, count(jsonb_array_elements(attendees) ->> 'username')
from events
where jsonb_array_elements(attendees) ->> 'username' like 'a%'
group by event_id
would obviously not work. Is there something that would return this output (count of usernames that begin with j for each event):
| event_id | count |
|----------|-------|
| 1 | 2 |
| 2 | 1 |
| 3 | 1 |

Answer 1

Well, just split your SQL logic to two part.
As below, you can get the all username for each event_id,
select
event_id,
jsonb_array_elements(attendees) ->> 'username' as user_name
from
events;
event_id | user_name
----------+-----------
1 | jsmith
1 | jjones
1 | swoods
2 | awilliams
2 | jlee
2 | bthomas
3 | dhanes
3 | speters
3 | jkay
(9 rows)
And then we can calculate some statistics data of json elements for event_id dimension,for example, you want to get the username's number of each event_id whose username started with some character such as 'j', the complete SQL would be:
with tmp as (
select
event_id,
jsonb_array_elements(attendees) ->> 'username' as user_name
from
events
)
select
event_id,
count(1)
from
tmp
where
user_name like 'j%'
group by
event_id
order by
event_id;
event_id | count
----------+-------
1 | 2
2 | 1
3 | 1
(3 rows)