Normally multi-select drop-down fields have at least one value selected. It may be worthwhile to know how to query if a multi-valued drop-down field has no value selected. This is allowed unless that field is mandatory. This applies to both out-of-box as well as custom fields of that type.
In this example we created a custom field named: "c_SagiTestMultiValued". We associated it with the Defect artifact. We have a many defects in our project and we'd like to query only those defect without any selected value for this attribute.
Enter the Query String, in this case it is: (c_SagiTestMultiVaued contains "") .
This would be the query to use to retrieve the Defects without values in this attribute.