我很穷，但是我给分

SQL> select num from forums;       NUM
----------
        16
         5
         3
         2
         1
         2
         1
         1---------------
SQL>  select sum(num) from forums;  SUM(NUM)
----------
        31---------------
SQL> select stddev(all num) from forums;STDDEV(ALLNUM)
--------------
    5.08323575作一个所有列的和的平方根