I have a table with users and I pull out all the ids from there:

select userid from users

Now for each ID From this list, I need to do certain operations (with other tables using these IDs)

Tell me how to organize a cycle on SQL

Answer 1, Authority 100%

not necessarily looped on the cursors – there is still an old-kind while (for clarity, an example for the case when the user’s primary key field):

declare @id int = 0
While Exists (Select 1 From Users WHERE UserID & GT; = @ID)
Begin.
  SELECT TOP (1) @ID = UserID From Users WHERE UserID & GT; = @ID
  - Necessary operations
  SET @ID = @ID + 1
End.

Answer 2

Try to use cursor

Answer 3

I think you need connections .
And cycles are a procedural approach to which it should be avoided.

Answer 4

If without a cycle, you can not do at all, you can use the cursor:

declare @userid int
Declare User_Cursor Cursor for
SELECT UserID From Users
Open User_cursor
Fetch Next From User_Cursor
Into @userid
While @@ fetch_status = 0
Begin.
  - do something with current @userid
  Fetch Next From User_Cursor
  Into @userid
End.
Close user_cursor;
Deallocate user_cursor;