Big Query: Join single latest row from second table - mysql

Question

I have two tables. One is a list of Orders, and one is a list of Events.
For each Order, I want to join the single last Event that happened (using clicked_at) before the created_at of the Order.
I have tried numerous ways to get this to work and tried several other answers on Stack Overflow but I am struggling to return the correct data.
The sudo logic for the subquery in my mind is something like:
SELECT campaign, user_id, created_at
FROM `Events`
WHERE order.user_id = user_id AND clicked_at < order.created_at
ORDER created_at DESC
LIMIT 1
Please see the example data below:
# Orders
| order_id | user_id | created_at |
-----------------------------------
| 123 | abc | 2020-07-04 |
| 456 | abc | 2020-05-01 |
# Events
| campaign | keyword | user_id | clicked_at |
----------------------------------------------
| facebook | shoes | abc | 2020-07-03 |
| google | hair | abc | 2020-07-01 |
My desired result
# Orders with campaign attribution
| order_id | user_id | created_at | campaign | keyword |
---------------------------------------------------------
| 123 | abc | 2020-07-04 | facebook | shoes |
| 456 | abc | 2020-06-04 | null | null |
Thanks!
Alex

Answer 1

Below is for BigQuery Standard SQL
#standardSQL
SELECT a.*, campaign, keyword
FROM `project.dataset.orders` a
LEFT JOIN (
SELECT
ANY_VALUE(o).*,
ARRAY_AGG(STRUCT(campaign, keyword) ORDER BY clicked_at DESC LIMIT 1)[OFFSET(0)].*
FROM `project.dataset.orders` o
JOIN `project.dataset.events` e
ON o.user_id = e.user_id
AND clicked_at < created_at
GROUP BY FORMAT('%t', o)
)
USING(order_id)
if applied to sample data from our question - result is
Row order_id user_id created_at campaign keyword
1 123 abc 2020-07-04 facebook shoes
2 456 abc 2020-05-01 null null

Answer 2

with orders as (
select 123 as order_id, 'abc' as user_id, cast('2020-07-04' as date) as created_at union all
select 456, 'abc', '2020-05-01'
),
events as (
select 'facebook' as campaign, 'shoes' as keyword, 'abc' as user_id, cast('2020-07-03' as date) as clicked_at union all
select 'google', 'hair', 'abc', '2020-07-01'
),
logic as (
select
orders.order_id,
orders.user_id,
orders.created_at,
events.clicked_at,
events.campaign,
events.keyword,
row_number() over (partition by orders.order_id order by events.clicked_at desc) as rn
from orders
left join events
on orders.user_id = events.user_id and events.clicked_at < orders.created_at
)
select * except(rn)
from logic
where rn = 1