select t.*, (select sum(mycount) from tab2 where tab_id=:tid) as sumt2
from tab t
order by t.nam

select sum(mycount) 
from tab2 
order by tid

select t.*, (select sum(mycount) from tab2 where tab_id=t.id and uids=:uids) as sumt2
from tab t
order by t.nam

procedure TForm1.IBDataSetsumt2GetText(Sender: TField; var Text: string; DisplayText: Boolean);
begin
  if TField(Sender).Value = null then Text := CurrToStr(getSumt2(IBDataSet.FieldByName('id').AsInteger))
                                  else Text := TField(Sender).Value;
end

IBDataSet.Close + IBDataSet.Open

IBQ:=TIBQuery.Create

FreeAndNil(IBQ)

IBQ:=TIBQuery.Create
try
  // тут работаем
finally
  FreeAndNil(IBQ)
end;

select t.*
from tab t
order by t.nam

select t.*, (select sum(mycount) from tab2 where tab_id=:tid) as sumt2
from tab t
order by t.nam

update TAB
set
  ID = :ID,
  NAM = :NAM,
  PRICE = :PRICE,
  COUNT = :COUNT
where
  ID = :OLD_ID

insert into TAB
  (ID, NAM, PRICE, COUNT)
values
  (:ID, :NAM, :PRICE, :COUNT)

delete from TAB
where
  ID = :OLD_ID

Select 
  ID,
  NAM,
  PRICE,
  COUNT
from TAB 
where
  ID = :ID

where tab_id=t.id

where tab_id=t.id and uids=:uids

where tab_id=t.id and uids=3

select t.*, :uids

select t.*, (:uids) as uids

select t.*, ('value') as virtField

select 
	t.*
	, (5) as sumt2
from 
	tab t
where
	t.id = :id

  if DataSet.FieldByName('sumt2').Value = null
    then DataSet.FieldByName('sumt2virt').Value := [Получаю значение и подставляю]
    else DataSet.FieldByName('sumt2virt').Value := DataSet.FieldByName('sumt2').Value;