hi!

it depends on your data and the type of operation u r doing.

If u need to calculate a value for all the rows or for the maximum rows coming out of the source then go for a connected lookup.

Or,if it is not so then go for unconnectd lookup.

Specially in conditional case like,

we have to get value for a field 'customer' from order tabel or from customer_data table,on the basis of following rule:

If customer_name is null then ,customer=customer_data.ustomer_Id

otherwise  

customer=order.customer_name.

so in this case we will go  for unconnected lookup