Yeah they seem quite obvious really. Serious question: is this really the types of questions asked for a dev job interview? (was still interesting to see.)

>List all departments along with the number of people there (tricky - people often do an "inner join" leaving out empty departments)

inner join seems the non obvious way to do it really IMO.

    select 
    departments.name as "department name",  
    (select sum(salary) from employees where employees.departmentid = departments.departmentid) as "department total salary" 
    from departments