Postgres : Multiply length of array to count aggregate result Following command works fine :

select count(*) * 2 from rooms;


But if I am trying to do something like : 

select count(*) * array_length(students,1) from rooms;


where students is an array column in the table rooms. Following error is thrown :

ERROR: column "rooms.students" must appear in the GROUP BY clause or be used in an aggregate function
SQL state: 42803
Character: 32


What is the right way to get the desired result?
sum was an obvious choice : 

select sum(array_length(students,1)) from rooms;