>>Yes, very simple...
>>
>>

>>SELECT Client.Numero
>> FROM Client
>> LEFT JOIN Invoice ON Invoice.Numero=Client.NoInvoice AND Invoice.SecondCondition=True
>> LEFT JOIN Inventory ON Inventory.Numero=Client.NoInventory AND Inventory.SecondCondition=True
>>WHERE Invoice.Numero IS NOT NULL OR Inventory.Numero IS NOT NULL
>>

>
>This does not work.

Is this the full statement? or do you have additional conditions in the where clause? If so, make sure you put parenthesis around the two expressions:

WHERE (Invoice.Numero IS NOT NULL OR Inventory.Numero IS NOT NULL) AND ...whathever ...