I have a combination of tables that are used to described location flows of people. The main 'travel_flows' tables has a structure as seen in the first table below. The order of the flows is described by the 'order_id' column.
+-----------+------------+---------+-------------+----------+ | travel_id | purpose_id | name_id | location_id | order_id | +-----------+------------+---------+-------------+----------+ | 434 | 23 | 55 | 85 | 1 | | 212 | 43 | 55 | 45 | 2 | | 411 | 41 | 55 | 17 | 3 | | 148 | 23 | 32 | 32 | 1 | | 153 | 11 | 32 | 19 | 2 | +-----------+------------+---------+-------------+----------+
What I am trying to achieve using PostgreSQL 9.6 is to group the returned rows by 'name_id' in a binary origin(from)/destination(to) format based on 'location_id' and 'order_id' value, similar to the following table:
+-----------------+--------------+------------------+---------------+------------+----------------+ | from_purpose_id | from_name_id | from_location_id | to_purpose_id | to_name_id | to_location_id | +-----------------+--------------+------------------+---------------+------------+----------------+ | 23 | 55 | 85 | 43 | 55 | 45 | | 43 | 55 | 45 | 41 | 55 | 17 | | 23 | 32 | 32 | 11 | 32 | 19 | +-----------------+--------------+------------------+---------------+------------+----------------+
Is there any way to achieve this with a select statement?
You can do this with lead
window function.
select * from (
select purpose_id,name_id,location_id,
lead(purpose_id) over(partition by name_id order by order_id) as to_purpose_id,
lead(name_id) over(partition by name_id order by order_id) as to_name_id,
lead(location_id) over(partition by name_id order by order_id) as to_location_id
from tbl
) t
where to_purpose_id is not null and to_name_id is not null and to_location_id is not null
You can easily achieve this with Window Functions (see https://www.postgresql.org/docs/current/static/tutorial-window.html ), specifically, function lead()
:
with flow as (
select
name_id,
purpose_id as from_purpose_id,
lead(purpose_id) over w1 as next_purpose_id,
location_id as from_location_id,
lead(location_id) over w1 as next_location_id,
order_id
from travel_flows
window w1 as (partition by name_id order by order_id)
)
select
name_id, from_purpose_id, next_purpose_id, from_location_id, next_location_id
from flow
where next_purpose_id is not null
order by name_id, order_id
;
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