I have a problem ... subquery alias problem

Question

select *, case 
when lagg is NULL then 'This student studies the most time' 
when hours_studied < lagg then 'This student studies ' || 
cast(lagg - hours_studied as string) || 'fewer than other' end details 
from (select students.*, lag(hours_studied) 
      over (PARTITION by school order by hours_studied desc) lagg 
      from students)  

when I execute the whole code then this happens.

ERROR: subquery in FROM must have an alias HINT: For example, FROM (SELECT...) [AS] foo.

Answer 1

select *, case 
when lagg is NULL then 'This student studies the most time' 
when hours_studied < lagg then 'This student studies ' || 
cast(lagg - hours_studied as string) || 'fewer than other' end details 
from (select students.*, lag(hours_studied) 
      over (PARTITION by school order by hours_studied desc) lagg 
      from students) x