Postgres : lag and lead with special conditions

Question

Forgive what may be a silly question, but I'm not much of a database guru.

Here is my table :

id_data | val_no3 | id_prev | id_next
--------+---------+---------+----------
1       |         |         | 2
2       |  7      |         | 
3       |         | 2       | 4
4       |  5      |         | 
5       |         | 4       | 10
6       |         | 4       | 10
7       |         | 4       | 10
8       |         | 4       | 10
9       |         | 4       | 10
10      |  8      | 4       | 

In the table below :

• id_prev is the value of the id_data which precedes when val_no3 is null

• id_next is the value of the id_data which folow when val_no3 is null

And now i would like to have this one :

id_data | val_no3 | id_prev | id_next  | val_prev | val_next
--------+---------+---------+----------+----------+----------
1       |         |         | 2        |          | 7 
2       |  7      |         |          |          |    
3       |         | 2       | 4        | 7        | 5
4       |  5      |         |          |          |
5       |         | 4       | 10       | 5        | 8
6       |         | 4       | 10       | 5        | 8
7       |         | 4       | 10       | 5        | 8
8       |         | 4       | 10       | 5        | 8
9       |         | 4       | 10       | 5        | 8
10      |  8      |         |          |          |

The conditions are as follows:

If val_no3 is null then : val_prev and val_next must be null

If val_no3 is not null then :

• val_prev must be equal to the previous value of val_no3 (it should be null if val_no3 which precedes is null too)

• val_next must be equal to the following value of val_no3 (it should be null if val_no3 which folows is null too)

---

I think i might have to use something with lag and lead but i don't know how to do.

I would be very grateful if you could give me your help to resolve this issue, thank you.

Answer 1

No need for analytic functions, just sub-selects. Something like the following (untested) should work:

select
  id_data,
  val_no3,
  id_prev,
  id_next,
  (select val_no2 from b where id_data = x.id_prev) as val_prev,
  (select val_no2 from b where id_data = x.id_next) as val_next
from
  b x
order by
  id_data;