If you want to be shure that a domain account was scanned for the computers, make an INNER JOIN to
tblADcomputers.
In order to find computers which are member of or have a function in a domain, use
tblComputersystem.DomainRole.
The first option (inner join to tblADComputers):
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetUnique,
tblAssets.Domain,
tsysOS.OSname,
tsysOS.Image As icon,
tblAssets.Lastseen
From tblAssets
Inner Join tblComputersystem On tblAssets.AssetID = tblComputersystem.AssetID
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysOS On tblAssets.OScode = tsysOS.OScode
Inner Join tblADComputers On tblAssets.AssetID = tblADComputers.AssetID
Where tblAssetCustom.State = 1
Order By tblAssets.AssetName
The second option (using tblComputersystem.DomainRole):
Select Top 1000000 tblAssets.AssetID,
tblAssets.AssetUnique,
tblAssets.Domain,
tsysOS.OSname,
tsysOS.Image As icon,
tblAssets.Lastseen
From tblAssets
Inner Join tblComputersystem On tblAssets.AssetID = tblComputersystem.AssetID
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Inner Join tsysOS On tblAssets.OScode = tsysOS.OScode
Where tblComputersystem.Domainrole > 0 And tblAssetCustom.State = 1
Order By tblAssets.AssetName