Condition lead results in postgres query - postgresql

Question

I have a table person_updates in postgresql with rows like:
| id | status | person_id | modified_at |
|----|--------|-----------|------------------|
| 1 | INFO | 2 | 2019-11-01 10:00 |
| 1 | UPDATE | 2 | 2019-11-02 15:00 |
| 1 | DEBUG | 2 | 2019-11-03 12:00 |
| 3 | INFO | 4 | 2019-11-04 14:00 |
| 3 | UPDATE | 4 | 2019-11-05 16:00 |
| 5 | INFO | 6 | 2019-11-06 08:00 |
| 5 | DEBUG | 6 | 2019-11-07 07:00 |
I want to get the INFO rows that are followed by an UPDATE row:
| id | status | person_id | modified_at |
|----|--------|-----------|------------------|
| 1 | INFO | 2 | 2019-11-01 10:00 |
| 3 | INFO | 4 | 2019-11-04 14:00 |
I've attempted this by doing a lead query
select d2.id, d2.status, d2.modified_at, d2.person_id,
lead(d2.status) over (partition by d2.id order by d2.modified_at) as next_status
from person_updates d2
where d2.status = 'INFO'
This returns more rows than I want. Adding a and d2.next_status = 'UPDATE' throws an error. How do I do this query?

Answer 1

Like this:
select t.id, t.status, t.modified_at, t.person_id
from (
select *,
lead(status) over (partition by id order by modified_at) as next_status
from person_updates
) t
where t.status = 'INFO' and t.next_status = 'UPDATE'
See the demo.
Results:
| id | status | modified_at | person_id |
| --- | ------ | ------------------------ | --------- |
| 1 | INFO | 2019-11-01T10:00:00.000Z | 2 |
| 3 | INFO | 2019-11-04T14:00:00.000Z | 4 |

Answer 2

You can use window function lead() to get the status of the next record. Since window functions are not allowed in the where clause, you need to turn the query to a subquery, and then filter in the outer query, like so:
select *
from (
select
t.*,
lead(status) over(partition by id order by modified_at) lead_status
from person_updates t
) t
where status = 'INFO' and lead_status = 'UPDATE'