Microsoft Query - How to show all results if paramter field left blank

Hi,

If someone could help me with this I would be very grateful;

I have three search boxes in XL , two are search from and to dates and the other is search be ref.

I can currently search by dates and ref but would like to show all records in the specified dates if the ref field is left blank.

I am using the query writer and referencing back to cells in XL. I don't use the wizard or SQL. I have tried many options and think I should be using a OR statement with a wildcard but can't seem to get the result, does the wildcard just show the blank cells in the actual data?

Many thanks for any help