多行函数:(聚合函数/分组函数) 解释:多条数据进入,单条结果出来(多进单出) 1).max(obj):最大值 2).min(obj):最小值 3).sum(num):求和 4).avg(num):求平均值 5).count(obj):计数 【注意事项】: 1).max()和min()两个函数可以接受任何数据类型的实际参数 2).sum()和avg()两个函数只能接受number类型的数据 3).多行函数/聚合函数/分组函数满足自动忽略空值的特点(在某些情况下,我们不应该忽略空值...) 案例如下: 查询公司薪资最高的、最低的、工资总和以及平均值的信息? select max(salary),min(salary),sum(salary),avg(salary) from employees; 参看如下代码并思考: select max(last_name),max(hire_date),min(last_name),min(hire_date) from employees; 关于count()的使用: 需求如下: 查询公司有多少员工? select count(employee_id),count(last_name),count(hire_date) from employees; select count(1),count(2),count(0),count(107),count('*') from employees; 执行以上代码发现效果都是正确的,我们以后做计数操作的时候,我们都用count('*')来实现; 查看如下代码: select count(department_id),count(commission_pct) from employees; 执行以上代码发现问题所在,只要是多行函数/聚合函数/分组函数满足自动忽略空值的特点 修改以上代码实现需要的效果: select count(nvl(department_id,100)),count(nvl(commission_pct,1)) from employees; 思考:avg() = sum() / count()? 答:以上的等式成立 需求如下: 查询公司的平均奖金率? select avg(commission_pct),sum(commission_pct) / count(commission_pct), sum(commission_pct) / count(nvl(commission_pct,2)), sum(commission_pct) / 107, sum(commission_pct) / count(*) from employees; 作业: --1.显示系统时间(注:日期+时间) select to_char(sysdate,'yyyy/mm/dd hh24:mi:ss') from dual; --2.查询员工号,姓名,工资,以及工资提高百分之20%后的结果(new salary) select employee_id,last_name,salary,salary * 1.2 "new salary" from employees; --3.将员工的姓名按首字母排序,并写出姓名的长度(length) select last_name,length(last_name) from employees order by last_name; --4.查询各员工的姓名,并显示出各员工在公司工作的月份数(worked_month)。 select last_name,round(months_between(sysdate,hire_date),0) "worked_month" from employees; --5.查询员工的姓名,以及在公司工作的月份数(worked_month),并按月份数降序排列 select last_name,round(months_between(sysdate,hire_date),0) "worked_month" from employees order by "worked_month" desc; --方式一: select last_name || ' earns $' || salary || ' monthly but wants $' || 3 * salary "Dream Salary" from employees; --方式二: select last_name || ' earns' || to_char(salary,'$99999') || ' monthly but wants' || to_char(3 * salary,'$99999') "Dream Salary" from employees; select last_name "Last_name",job_id "Job_id", decode(job_id,'AD_PRES','A', 'ST_MAN','B', 'IT_PROG','C', 'SA_REP','D', 'E') "Grade" from employees where job_id in('AD_PRES','ST_MAN','IT_PROG','SA_REP','ST_CLERK') order by "Grade" desc; select last_name "Last_name",job_id "Job_id", case job_id when 'AD_PRES' then 'A' when 'ST_MAN' then 'B' when 'IT_PROG' then 'C' when 'SA_REP' then 'D' else 'E' end "Grade" from employees where job_id in('AD_PRES','ST_MAN','IT_PROG','SA_REP','ST_CLERK') order by "Grade" desc;