select inv.id,
invoicenum,
datetime,
inv.discpercent,
discamount,
nettotal, 
taxtotal,
total,
printed,
inv.per_id as recipient_per_id,
rtrim(recipper.lastname || ', ' || 
coalesce(recipper.firstname,'') || ' ' ||
coalesce(recipper.title,'')) as _recipient,
inv.clinic_id,
clinic.name as _clinicname,
inv.staff_id,
staff.knownas as _staffmember,
inv.room_id,
room.name as _roomname,
inv.apptype_id,
apptype.name as _apptype,
inv.patient_per_id,
rtrim(patper.lastname || ', ' || 
coalesce(patper.firstname,'') || ' ' ||
coalesce(patper.title,'')) as _patient,
patper.code, patper.insuranceref, (coalesce(clinic.prefix, '') || 
cast(inv.invoicenum as varchar(20))) as _invoiceref,
coalesce(view_invamtalloc.amtalloc, 0) as amtpaid,
case
 when (inv.total = 0) or
      (view_invamtalloc.amtalloc >= inv.total) then 'Paid In Full'
 when (view_invamtalloc.amtalloc > 0) and
      (view_invamtalloc.amtalloc < inv.total) then 'Part Paid'
 else 'Unpaid'
end as _status
from inv
inner join per as recipper on (inv.per_id = recipper.id)
inner join clinic on (inv.clinic_id = clinic.id)
left join staff on (inv.staff_id = staff.id)
left join room on (inv.room_id = room.id)
left join apptype on (inv.apptype_id = apptype.id)
left join per as patper on (inv.patient_per_id = patper.id)
left join view_invamtalloc on (inv.id = view_invamtalloc.inv_id)

View definition:

create view view_invamtalloc AS
select inv_id, per_id, sum(amount) as amtalloc
from alloc
where (inv_id is not null)
group by inv_id, per_id;