avg 求平均
count 求數量
max 求值可針對數字,日期,字符
min 求值可針對數字,日期,字符
sum 求總和
listagg
stddev
variance
count詳解:
count(*)將返回表格中所有存在的行的總數包括值為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(有默認值的列也會被計入).
distinct 列名,得到的結果將是除去值為null和重復數據后的結果
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,要用having子句才能加分組函數
alias 別名不能放在group by 子句中
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