Average of successive pairs of rows - sql

Question

I have a table like so:
id | value
---+------
1 | 10
2 | 5
3 | 11
4 | 8
5 | 9
6 | 7
The data in this table is really pairs of values, which I need to take the average of, which should result in:
pair_id | pair_avg
--------+---------
1 | 7.5
2 | 9.5
3 | 8
I have got some other information (a pair of flags) which could also help to pair them, though they still have to be in id order. I cannot really change how the data comes to me.
As I'm more used to arrays than SQL, all I can think is that I need to loop through the table and sum the pairs. But this doesn't strike me as very SQL-ish.
Update
In making this minimal example, I have apparently over simplified.
As the table I am working with is the result of several selects, the IDs will not be quite so clean, apologies for not specifying this.
The table looks a lot more like:
id | value
----------
1 | 10
4 | 5
6 | 11
7 | 8
10 | 9
15 | 7
The results will be used to create a second table, I don't care about the index on this new table, it can provide its own, therefore giving the result already indicated above.

Answer 1

If your data is as clean as the question makes it seem: no NULL values, no gaps, pairs have consecutive positive numbers, starting with 1, and assuming id is type integer, it can be as simple as:
SELECT (id+1)/2 AS pair_id, avg(value) AS pair_avg
FROM tbl
GROUP BY 1
ORDER BY 1;
Integer division truncates the result and thus takes care of grouping pairs automatically this way.
If your id numbers are not as regular but at least strictly monotonically increasing like your update suggests (still no NULL or missing values), you can use a surrogate ID generated with row_number() instead:
SELECT id/2 AS pair_id, avg(value) AS pair_avg
FROM (SELECT row_number() OVER (ORDER BY id) + 1 AS id, value FROM tbl) t
GROUP BY 1
ORDER BY 1;
db<>fiddle here

Answer 2

I think you can just use group by with arithmetic:
select row_number() over (order by min(id)), min(id), max(id), avg(id)
from t
group by floor( (id - 1) / 2 );
I'm not sure why you would want to renumber the ids after aggregation. The original ids seem more useful.

Answer 3

You may use ceil function by appliying division by 2 to id column as in the following select statement :
with t(id,value) as
(
select 1 , 10 union all
select 2 , 5 union all
select 3 , 11 union all
select 4 , 8 union all
select 5 , 9 union all
select 6 , 7
)
select ceil(id/2::numeric) as "ID", avg(t.value) as "pair_avg"
from t
group by "ID"
order by "ID";
id | pair_avg
-------------
1 | 7.5
2 | 9.5
3 | 8