I think the sql is being broken up like this:

--this select statement has 3 columns

select staff.staffid, staff.staffname, staff.department from staff left join department on staff.departmentid = department.departmentid

union all

--this select statement has 2 columns

select department.departmentname, department.domain from department left join staff on department.departmentid = staff.departmentid;

A union should be between selects with the same number of columns