Thread: calculating the sum of a field in a query with join

Select attend.overtimehours from attendance attend, salarydetail sal where attend.employeeid = sal.employeeid

in the above query,how can i fetch the sum of overtimehours?
attendance table
column_name data_type
-------------------- -------------------
attendancerid number
employeeid number
shiftid number
dayofattendance date
timein date
timeout date
numberofhours number
overtimehours number
presentflag varchar2

salarydetail table
column_name data_type
----------------- --------------------
salaryid number
employeeid number
startdate date
enddate date
grosssalary number
netsalary number
overtimeamount number
lopamount number
paiddays number