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SQL - Select unique row with average of all other rows
UserID     Age
    1          22
    2          34
    3          23
    4          19
    etc.

UserID      Age      AvgAge
   1           22       24.5

Select UserID, Age, Avg(Age) as 'AvgAge'
  From Users
  Where UserID = 1
  Group By UserId, Age

  UserID      Age      AvgAge    //Result set
  1           22       22

declare @T table
(
  UserID int,
  Age int
)

insert into @T values
    (1,          22),
    (2,          34),
    (3,          23),
    (4,          19)

declare @UserID int = 1

select Age, (select avg(Age*1.0)
             from @T
             where UserID <> @UserID) as AvgAge
from @T
where UserID = @UserID

Age         AvgAge
----------- ---------------------------------------
22          25.333333

SELECT u.UserID,
        u.Age,
        (SELECT AVG(uavg.Age)
           FROM Users uavg
          WHERE uavg.UserID != u.UserID) AS AvgAge
    FROM Users u
   WHERE u.UserID = 1

Select
 UserID,
 Age,
 (select Max(Age) from Users) as 'AvgAge'
  From Users
  Where UserID = 1

Select U.UserID, u.Age, sq.Age as 'AvgAge'
      From Users u
      join (select average(age) as Age from users) sq on 1=1
      Where UserID = 1
      Group By UserId, Age

SELECT
    u.UserId,
    u.Age,
    b.AvgAge
FROM
    dbo.Users a,
    (SELECT AVG(Age*1e0) as AvgAge FROM dbo.Users) as b

declare @T table (UserID int, Age int)
insert into @T values(1,22),(2,34),(3,23),(4,19)

declare @UserID int = 1

;with a as
(
  select userid, Age,
  avg(age * case when userid <> @userid then 1.0 end) over() 'AvgAge'
from @T
)
select Age, AvgAge from a
where userid = @UserID