Replying to my own question, as I have figured out a working solution. Not sure if there is a better way to do this, but if there is I'm all ears.
Anyways here is my report:
Select Distinct Top 1000000 tblAssets.AssetID,
tblAssets.AssetName,
tsysAssetTypes.AssetTypename As [Phone Type],
tsysAssetTypes.AssetTypeIcon10 As icon,
tblAssetCustom.Manufacturer,
tblAssetCustom.Model,
tblAssetCustom.Custom2 As Carrier,
tblAssetCustom.Custom3 As [Phone Num],
tblAssets.Lastseen
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysAssetTypes On tsysAssetTypes.AssetType = tblAssets.Assettype
Full Join (Select Top 100000 tblAssetUserRelations.RelationID,
tblAssetUserRelations.AssetID,
tblAssetUserRelations.EndDate
From tblAssetUserRelations
Where tblAssetUserRelations.EndDate Is Not Null) SubTest1 On
tblAssets.AssetID = SubTest1.AssetID
Where tsysAssetTypes.AssetTypename = 'iPhone' And tblAssetCustom.State = 1 And
(Select COUNT(*) From tblAssetUserRelations
Where tblAssetUserRelations.AssetID = tblAssets.AssetID And
tblAssetUserRelations.EndDate Is Null) < 1
Order By tblAssets.AssetName
I just do a Select COUNT(*) and specify that I only want Active, iPhones, with less than 1 user relationship with no EndDate. If there is one relationship with no EndDate then we know the phone is still in use.