分组函数Group functions
avg 求平均 count 求数量 max 求值可针对数字，日期，字符 min 求值可针对数字，日期，字符 sum 求总和 listagg stddev variance
count详解：
count(*)将返回表格中所有存在的行的总数包括值为null的行，然而count(列名)将返回表格中除去null以外的所有行的总数(有默认值的列也会被计入）.
distinct 列名,得到的结果将是除去值为null和重复数据后的结果
select avg(salary),max(salary),min(salary),sum(salary) from employees where job_id like '%REP%';
select count(*) from employees where department_id=50;
select count(commission_pct) from employees where department_id=50; 空值行不参与计算
GROUP BY 分组 select department_id,avg(salary) from employees group by department_id;
select department_id,avg(salary) from employees group by department_id; order by avg(salary); 此group by 后可跟order by ，但order by子句只能出现在最后。
select avg(salary) from employees group by department_id; 但group by 的列名不必一定在select 子句中
SELECT department_id, AVG(salary) FROM employees GROUP BY department_id HAVING AVG(salary) > 8000;
where 子句不能加分组函数，要用having子句才能加分组函数 alias 别名不能放在group by 子句中
