PostgreSQL previous and next group value - sql

Question

The problem is the following:
Suppose, I have a table of such view (it is a sub-sample of the table I'm working with):
| col1 | col2 |
|------|------|
| 1 | a2 |
| 1 | b2 |
| 2 | c2 |
| 2 | d2 |
| 2 | e2 |
| 1 | f2 |
| 1 | g2 |
| 3 | h2 |
| 1 | j2 |
I need to add two new columns
prev containing the previous value in col1 not equal to the current
next containing the next value in col1 not equal to the current
If there is no previous value, prev should contain the current col1's value as well as next should contain the current value if no next values exist.
Result should have the following form:
| col1 | col2 | prev | next |
|------|------|------|------|
| 1 | a2 | 1 | 2 |
| 1 | b2 | 1 | 2 |
| 2 | c2 | 1 | 1 |
| 2 | d2 | 1 | 1 |
| 2 | e2 | 1 | 1 |
| 1 | f2 | 2 | 3 |
| 1 | g2 | 2 | 3 |
| 3 | h2 | 1 | 1 |
| 1 | j2 | 3 | 1 |
I will be grateful any help.

Answer 1

You can try this using combination of window functions lead, lag, first_value, last_value and sum.
select
t.col1, t.col2, n,
coalesce(first_value(y) over (partition by x order by col2), col1) prev_val,
coalesce(last_value(y2) over (partition by x order by col2
rows between current row and unbounded following), col1) next_val
from (
select
t.*,
case when col1 <> lag(col1) over (order by col2) then lag(col1) over (order by col2) end y,
case when col1 <> lead(col1) over (order by col2) then lead(col1) over (order by col2) end y2,
sum(n) over (order by col2) x
from (
select
t.*,
case when col1 <> lag(col1) over (order by col2) then 1 else 0 end n
from t
) t
) t;
It finds the lead/lag per group of rows.

Answer 2

If I assume that you have an id column that specifies the ordering, then this is possible. I'm just not sure this is easily expressed using window functions.
You can use correlated subqueries:
select t.*,
(select t2.col1
from t t2
where t2.id < t.id and t2.col1 <> t.col1
order by t2.id desc
fetch first 1 row only
) as prev_col1,
(select t2.col1
from t t2
where t2.id > t.id and t2.col1 <> t.col1
order by t2.id asc
fetch first 1 row only
) as prev_col2
from t;
You can add the coalece() for missing previous and next values. That is not the interesting part of the problem.

Answer 3

WITH cte AS (
SELECT row_number() over() rowid, *
FROM unnest(array[1,1,2,2,2,1,1,3,1], array['a2','b2','c2','d2','e2','f2','g2','h2','j2']) t(col1,col2)
)
SELECT t.col1,
t.col2,
COALESCE(prev.col1,t.col1) prev,
COALESCE("next".col1,t.col1) "next"
FROM cte t
LEFT JOIN LATERAL (SELECT prev.col1
FROM cte prev
WHERE prev.rowid < t.rowid
AND prev.col1 != t.col1
ORDER BY prev.rowid DESC
LIMIT 1
) prev ON True
LEFT JOIN LATERAL (SELECT "next".col1
FROM cte "next"
WHERE "next".rowid > t.rowid
AND "next".col1 != t.col1
ORDER BY "next".rowid ASC
LIMIT 1
) "next" ON True