I am currently trying to create a report which should find all 'Printer' Assets, which aren't mapped to any asset location. The report underneath finds all printers, which already have a location set, though I can't seem to find the right criteria for when the asset location is undefined.
Select Top 1000000 tsysAssetTypes.AssetTypeIcon10 As icon,
  tblAssets.AssetName As Asset,
  tblAssets.Domain,
  tsysAssetTypes.AssetTypename As AssetType,
  tblAssets.Username,
  tblAssets.Userdomain,
  tblAssets.IPAddress,
  tblAssetCustom.Manufacturer,
  tblAssetCustom.Model,
  tblAssetCustom.Serialnumber,
  tblAssets.Firstseen,
  tblAssets.Lastseen,
  tblAssets.Lasttried,
  tsysAssetRelationTypes.Name As Relation,
  tblAssets1.AssetID,
  tblAssets1.AssetName,
  tblAssets1.Description
From tblAssets
  Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
  Inner Join tblAssetRelations On tblAssetRelations.ChildAssetID =
    tblAssets.AssetID
  Inner Join tsysAssetRelationTypes On tsysAssetRelationTypes.RelationTypeID =
    tblAssetRelations.Type
  Inner Join tblAssets tblAssets1 On
    tblAssets1.AssetID = tblAssetRelations.ParentAssetID
  Inner Join tblAssetCustom On tblAssetCustom.AssetID = tblAssets.AssetID
Where tsysAssetTypes.AssetTypename = 'Printer' And tsysAssetRelationTypes.Name =
  'is located in'
Order By tblAssets1.AssetName,
  tblAssets.Domain,
  Asset