The dbxquery command takes an SQL statement as an argument so your "SPL" is simply
| dbxquery query="SELECT Computer_System_DATA.Name00 as ComputerName, DetectionTime, Users.UserName, Process, ThreatName, Path, EP_ThreatSeverities.Severity, EP_ThreatCategories.Category, CleaningAction, ExecutionStatus, ActionSuccess, PendingActions, ErrorCode, RemainingActions, LastRemainingActionsCleanTime FROM dbo.EP_Malware INNER JOIN dbo.Computer_System_DATA on EP_Malware.MachineID = Computer_System_DATA.MachineID INNER JOIN dbo.EP_ThreatCategories on EP_Malware.CategoryID = EP_ThreatCategories.CategoryID INNER JOIN dbo.EP_ThreatSeverities on EP_Malware.SeverityID = EP_ThreatSeverities.SeverityID INNER JOIN dbo.Users on EP_Malware.UserID = Users.UserID ORDER BY DetectionTime ASC"
However, IME DBX does not work well with complex SQL queries. I recommend creating a view with the needed joins and using DBX to read from the view.
... View more