Question

I am using oracle 10g and wrote a query like this:

SELECT presenttime,
       employeeid
  FROM mobilelocation
 WHERE presentdate = '31-10-2011'
 GROUP BY employeeid;

resulting an error message

ORA-00979: not a GROUP BY expression

But upon removing this group clause returining the result without any errors like this,

PRESENTTIME EMPLOYEEID
23:25   12304
23:48   12305

What is the problem with my group by clause and also suggest me how to use order by clause also

Answer 1

GROUP BY implies that you want to aggregate multiple rows from the table into a single row for every EmployeeID value. If that is your goal, you’d need to specify an aggregate function on the columns that you are not grouping by. If you want the earliest PresentTime value for each EmployeeID, for example, you’d do something like

SELECT EmployeeID, MIN(PresentTime) earliestPresentTime
  FROM mobileLocation
 WHERE PresentDate = date '2011-10-31'
 GROUP BY EmployeeID