>This would work but I am wondering if there could be something simpler:
>
>

>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 (EXISTS (SELECT Invoice.Numero FROM Invoice WHERE Invoice.Numero=Client.NoInvoice AND Invoice.SecondCondition=True) OR
>  EXISTS (SELECT Inventory.Numero FROM Inventory WHERE Inventory.Numero=Client.NoInventory AND Inventory.SecondCondition=True))
>

Try this ?

select Client.Numero from Client join Invoice ON Invoice.Numero=Client.NoInvoice AND Invoice.SecondCondition=True 
union 
select Client.Numero from Client join Inventory ON Inventory.Numero=Client.NoInventory AND Inventory.SecondCondition=True