组函数针对的是指定字段的非空值。注意：where子句中不能出现组函数！！！
id name dept_id salary manager_id0001 wangda 101 8500[NULL]0002 wanger 1033 000 00090003 wangsan 102 4000 00060004 wangsi 104 2000 00050005 wangwu 104 6000 00010006 wangliu 102 7000 00010007 wangqi 105 5000 00080008 wangba 105 5500 00010009 wangjiu 103 6000 00010010 wangshi 104 900 00050011 wangsy 103 5000 00090012 wangse [NULL]3000[NULL]SELECT dept_id, avg(salary)FROM personGROUP BY dept_id;dept_id avg(salary)[NULL] 3000.000000101 8500.000000102 5500.000000103 4666.666667104 2966.666667105 5250.000000SELECT p.dept_id, dept_name, avg(salary)FROM person p,dept dwhere p.dept_id = d.dept_idGROUP BY p.dept_idHAVING avg(salary)>5000;dept_id dept_name avg(salary)101 zongwu 8500.000000102 zhenggong 5500.000000105 renshi 5250.000000原文：http://www.cnblogs.com/rainyeagle/p/4599461.html