Query/report filter

How can I add a filter to a query/report without making it mandatory? For example, if I add a filter, I want it to be optional, not mandatory.

Are you asking for custom report? if yes you can just remove "reqd": 1 from the js file of that report.
Example:

{
	"fieldname":"customer",
	"label": __("Customer"),
	"fieldtype": "Link",
	"options": "Customer",
 // "reqd": 1,   Remove this key and value.

}