SQL: Calculate Average Device Time with Conditional Aggregation

SELECT toStartOfDay(toDateTime(Ds)) AS datetime_day_Ds, AppIdentifier AS AppIdentifier, CASE WHEN count() > 1 THEN sum(AvgDeviceTime) / count() ELSE sum(AvgDeviceTime) END AS index_avgdevicetime_0 FROM table_name GROUP BY datetime_day_Ds, AppIdentifier