SQL query to get average sum of other rows and store in current rows - sql

Question

I have table like this, and I want to query to store the average of others row points.
USER_ID POINTS
------------- --------
a14e43e4f851 134
1e86e5adedbf 40
3c66730edf69 149
32e24082f97b 67
b33e3100a7be 124
274ee414ad8f 85
bdeef25fc797 172
For example - for user_id = a14e43e4f851, the average sum of points should be
avg(40+149+67+124+85+172) .
PS - not taken the points (134) in calculation for user a14e43e4f851.
Output should look like this --
USER_ID POINTS AVG
------------- ------- ------
a14e43e4f851 134 106 which is avg(40+149+67+124+85+172)
1e86e5adedbf 40 avg(134+149+67+124+85+172)
3c66730edf69 149 avg(134+40+67+124+85+172)
32e24082f97b 67 avg(134+40+149+124+85+172)
b33e3100a7be 124 ...
274ee414ad8f 85 ...
bdeef25fc797 172 ...

Answer 1

You could use a correlated subquery:
select t.*,
(select avg(t1.points) from mytable t1 where t1.user_id <> t.user_id) as average
from mytable t
An alternative uses window functions:
select t.*,
(sum(points) over() - points) / nullif(count(*) - 1, 0) as average
from mytable t
Note: avg obviously conflicts with a language keyword, I use average instead.
If you wanted an update statement:
update mytable t
set t.average = (
select avg(t1.points) from mytable t1 where t1.user_id <> t.user_id
)
However, I would not recommend actually storing this value; this is derived information, that can easily be computed on the fly whenever needed, using the first statement. If you are going to run the query often, you could create a view:
create view myview as
select t.*,
(sum(points) over() - points) / nullif(count(*) - 1, 0) as average
from mytable t

Answer 2

I'm assuming user_id is the PK.
WITH q AS (SELECT sum(points) AS s, count(*) AS n FROM mytable)
UPDATE table SET average = (q.s-points)/(q.n-1);
The idea is that
average of all other user's score is sum(score)/count(*)
sum of all user scores except this one is equal to sum of all scores minus this user's score
average of all other users' score except this one is (sum(score)-score_for_this_user)/(count(*)-1)
Nice thing is it only has to calculate the sum() and count() once.
To handle the case where there is only one row in the table:
WITH q AS (SELECT sum(points) AS s, NULLIF(count(*),0) AS n FROM mytable)
UPDATE table SET average = (q.s-points)/(q.n-1);
This makes the count NULL instead of 0, so the average updated should be NULL too.