分組函數(shù)Group functions

avg 求平均
count 求數(shù)量
max 求值可針對數(shù)字，日期，字符
min 求值可針對數(shù)字，日期，字符
sum 求總和
listagg
stddev
variance

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count詳解：

count(*)將返回表格中所有存在的行的總數(shù)包括值為null的行，然而count(列名)將返回表格中除去null以外的所有行的總數(shù)(有默認(rèn)值的列也會被計入）.

distinct 列名,得到的結(jié)果將是除去值為null和重復(fù)數(shù)據(jù)后的結(jié)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;
空值行不參與計算

GROUP BY分組
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子句只能出現(xiàn)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 子句不能加分組函數(shù)，要用having子句才能加分組函數(shù)
alias 別名不能放在group by 子句中