分组计算函数和GRPOUP BY子句
分组计算函数
函数 | |
---|---|
求和 | SUM |
求平均值 | AVG |
计数 | COUNT |
求标准差 | STDDEV |
求方差 | VARIANCE |
求最大值 | MAX |
求最小值 | MIN |
示例:
SELECT AVG(salary), MAX(salary), MIN(salary), SUM(salary)
FROM employees
WHERE job_id LIKE '%REP%';
SELECT MIN(hire_date), MAX(hire_date) FROM employees;
COUNT函数
函数用法 | 说明 |
---|---|
count(*) | 返回满足选择条件的所有行的行数,包括值为空的行和重复的行 |
count(expr) | 返回满足选择条件的且表达式不为空行数 |
count(DISTINCT expr) | 返回满足选择条件的且表达式不为空,且不重复的行数 |
示例
select count(*) from employees e;
select count(1) from employees e;
所有行数(上面俩个结果相同,1与*一样)
select count(e.department_id) from employees e;
所有不为空的行数
select count(distinct e.department_id) from employees e;
所有不为空且不重复的行数
当分组计算函数遇到NULL
select (select sum(commission_pct) from employees) /
(select count(*) from employees)
from dual
等价于
SELECT AVG(NVL(commission_pct, 0)) FROM employees;
使用GROUP BY子句分组:
1、可以按照某一个字段分组,也可以按照多个字段的组合进行分组
SELECT AVG(salary), department_id FROM employees GROUP BY department_id;
SELECT department_id dept_id, job_id, SUM(salary)
FROM employees
GROUP BY department_id, job_id;
2、SELECT 查询语句中同时选择分组计算函数表达式和其他独立字段时 ,其他字段必须出现在Group By子
句中,否则不合法
SELECT department_id, count(last_name)
FROM employees
GROUP BY department_id;
3、不能在Where 条件中使用分组计算函数表达式,当出现这样的需求的时候,使用Having 子句
SELECT department_id, AVG(salary)
FROM employees
GROUP BY department_id
HAVING AVG(salary) > 8000;
4、分组计算函数也可嵌套使用
SELECT MAX(AVG(salary))
FROM employees
GROUP BY department_id;
注:获取最高的部门平均薪水
5、排序后取第一行的值
select * from (select count(to_char(e.hire_date,'DAY')) as dd
from employees e
group by to_char(e.hire_date,'DAY') order by dd) a where rownum=1
注:rownum可等于任何数,就是多少行