AVG

AVG([ DISTINCT | ALL ] expr) [ OVER(analytic_clause) ]

 SELECT MANAGER_ID,
           LAST_NAME,
           HIRE_DATE,
           SALARY,
           AVG(SALARY) OVER(PARTITION BY MANAGER_ID ORDER BY HIRE_DATE ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING) AS C_MAVG
      FROM EMPLOYEES
     WHERE MANAGER_ID BETWEEN 101 AND 108
     ORDER BY MANAGER_ID, HIRE_DATE, SALARY;

MANAGER_ID LAST_NAME         HIRE_DATE       SALARY     C_MAVG
---------- ------------------------- ----------- ---------- ----------
       101 Mavris                    2002/6/7       6500.00       8250
       101 Baer                      2002/6/7      10000.00 9502.66666
       101 Higgins                   2002/6/7      12008.00 11338.6666
       101 Greenberg                 2002/8/17     12008.00       9472
       101 Whalen                    2003/9/17      4400.00       8204
       102 Hunold                    2006/1/3       9000.00       9000
       103 Austin                    2005/6/25      4800.00       4800
       103 Pataballa                 2006/2/5       4800.00       4600
       103 Lorentz                   2007/2/7       4200.00       5000
       103 Ernst                     2007/5/21      6000.00       5100
       108 Faviet                    2002/8/16      9000.00       8600
       108 Chen                      2005/9/28      8200.00       8300
       108 Sciarra                   2005/9/30      7700.00       7900
       108 Urman                     2006/3/7       7800.00 7466.66666
       108 Popp                      2007/12/7      6900.00       7350
15 rows selected