How would you go about displaying a list of fields that a user can select to make a query? The list can get pretty long (we are not talking hundreds of items but it would still be a sore to the eye) I wanted to group the fields or make filtering with tags, but the problem is that there would be items that wouldn't belong to any group.
I've seen pages that use a radio button to select the group of list items that appear in the list. The radio buttons are restricted to a few broad categories, such as "Customer Fields () Item Fields () Tax Fields (x) All Fields ()". Selecting the radio button immediately populates the list box with only the fields relevant to the chosen topic.
The inclusion of an "All Fields" choice lets people know they're looking at every possible field you offer, which is a good way of reducing questions such as "Do you support querying on the CityParkingZone field?" Generally, users will also understand that the All Fields list will be quite long, and that they'll have to do more work to search through the list to find the fields they're interested in. It also avoids having a somewhat confusing category of "Miscellaneous fields" where you might be tempted to place the fields that didn't fit your other broad groups.
If you have specific groups, keep them together as a filter option. Anything outside of that can be captured using the "Search" functionality.
If the number of groups is an eyesore, you can try using Progressive Disclosure techniques to show the extra functionality on demand.