Add top 1000000
Select top 1000000 tblCPlogoninfo.Username, tblCPlogoninfo.Domain,
Count(tblCPlogoninfo.Computername) As total
From tblCPlogoninfo
Where tblCPlogoninfo.logontime > GetDate() - 30
Group By tblCPlogoninfo.Username, tblCPlogoninfo.Domain
Order By Count(tblCPlogoninfo.Computername) Desc