How to lag and get first value base on condition on bigquery sql?

Question

Hi I have a table and want to return the session_id where rn=1 for each customer, rn is not always the same. rn here is row_number over (partition by customer_id, order_id by date_key) rn

customer_id	session_id	order_id	date_key	rn
1	2342	A123	2022-01-01	1
1	2343	A125	2022-01-01	2
1	2344	A126	2022-01-01	3
2	2345	A127	2022-01-01	1
2	2346	A128	2022-01-01	2
2	2346	A129	2022-01-01	3
2	2346	A130	2022-01-01	4

customer_id	session_id	order_id	date_key	rn	firstSession
1	2342	A123	2022-01-01	1	2342
1	2343	A125	2022-01-01	2	2342
1	2344	A126	2022-01-01	3	2342
2	2345	A127	2022-01-01	1	2345
2	2346	A128	2022-01-01	2	2345
2	2346	A129	2022-01-01	3	2345
2	2346	A130	2022-01-01	4	2345

I could basically do a

with firstsession as
(select * from table1 where rn=1)

select 
 t1.* 
 ,fs.session_id firstSession
from table1 t1
left join 
   firstsession fs 
   on t1.customer_id=fs.customer_id and t1.date_key=fs.date_key 

How do I do a lag function to get the same result?

Answer 1

Consider below approach

select *, 
  first_value(session_id) over session as firstSession
from your_table
window session as (partition by customer_id order by date_key) 

     

if applied to sample data in your question - output is