How i prevent customer invoice no field data exist from updated when i transfer empty data to same field

Hi guys can any one help me in this task

I create field customer invoice no in Database B and Database A to give every group of sales invoice line unique number

this processes done IN Database B customer invoice no is sales invoice line table when this field is empty in same sales invoice no it remove value that found in database B

ex

Database A Database B

Sales invoice line table Sales invoice line table

No customer invoice no No customer invoice no

so-30 so-30 300

When i transfer data from Database A to database B in table sales invoice line

it clear or remove 300 and become no value

How i put condition by code tell him

when import

if sales invoice line table .customer invoice no field has value

not updated

Are this possible by code please help me

thanks.

Put a filiter on Field where value is blank

and update only those record …

Are this true

In before import trigger of this table i want to transfer it i write as following

setrange (CustomerInvoiceNo,"");

if find first

sales invoice line.modify