>>But what exactly do you want to achieve here?
>>

>>select TABLE1.*, SumInfo.Used_Quantity FROM TABLE1 
>>INNER JOIN (select TABLE2.ID_FLD, SUM(
>> TABLE2.QUANTITY)  as USED_QUANTITY FROM TABLE2 GROUP BY TABLE2.ID_FLD) SumInfo 
>>on Table1.ID_Fld = SumInfo.ID_Fld

>>
>>If you want to check FLD IN (SubSelect), then SubSelect MUST return only one field.
>
>Thank you. I think you are right. I will try your suggestion.

You're welcome. The technique is called "derived tables" technique.