with t1 as (

SELECT  *,
lag(timestamp) over(partition by user_id order by timestamp) as timestamp_lag,
if(TIMESTAMP_DIFF(timestamp, lag(timestamp) over(partition by user_id order by timestamp),minute)<30,0,1) as session_start
FROM `table_name`

)

select *,
sum(session_start) over(partition by user_id order by timestamp) as session_number
from t1