分組計算函數和GRPOUP BY子句
分組計算函數
函數 | |
---|---|
求和 | SUM |
求平均值 | AVG |
計數 | COUNT |
求標準差 | 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函數
函數用法 | 說明 |
---|---|
count(*) | 返回滿足選擇條件的所有行的行數,包括值爲空的行和重複的行 |
count(expr) | 返回滿足選擇條件的且表達式不爲空行數 |
count(DISTINCT expr) | 返回滿足選擇條件的且表達式不爲空,且不重複的行數 |
示例
select count(*) from employees e;
select count(1) from employees e;
所有行數(上面倆個結果相同,1與*一樣)
select count(e.department_id) from employees e;
所有不爲空的行數
select count(distinct e.department_id) from employees e;
所有不爲空且不重複的行數
當分組計算函數遇到NULL
select (select sum(commission_pct) from employees) /
(select count(*) from employees)
from dual
等價於
SELECT AVG(NVL(commission_pct, 0)) FROM employees;
使用GROUP BY子句分組:
1、可以按照某一個字段分組,也可以按照多個字段的組合進行分組
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 查詢語句中同時選擇分組計算函數表達式和其他獨立字段時 ,其他字段必須出現在Group By子
句中,否則不合法
SELECT department_id, count(last_name)
FROM employees
GROUP BY department_id;
3、不能在Where 條件中使用分組計算函數表達式,當出現這樣的需求的時候,使用Having 子句
SELECT department_id, AVG(salary)
FROM employees
GROUP BY department_id
HAVING AVG(salary) > 8000;
4、分組計算函數也可嵌套使用
SELECT MAX(AVG(salary))
FROM employees
GROUP BY department_id;
注:獲取最高的部門平均薪水
5、排序後取第一行的值
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可等於任何數,就是多少行