[学习笔记] Oracle字符串函数、日期函数、数值函数、转换函数、聚合函数

函数

• 单行函数：对一行数据进行操作的函数，如字符串函数、数值函数、转换函数、日期函数等。
• 聚合函数：同时对多行数据进行操作，如求和函数等。

字符串函数

日期函数

select to_char(sysdate,'yyyy-mm-dd hh24:mi:ss') from dual;
select systimestamp from dual;
select dbtimezone from dual;

-- 结果为 2018-11-30，2018-10-31
select 
    to_char(add_months(to_date('2018-10-31','yyyy-mm-dd'),1),'yyyy-mm-dd'), 
    to_char(add_months(to_date('2018-09-30','yyyy-mm-dd'),1),'yyyy-mm-dd')
from dual;

select last_day(sysdate) from dual;

select next_day(to_date('2018-11-12','yyyy-mm-dd'),'星期四') from dual;

select extract(year from timestamp '2018-11-12 15:36:01') as year,
       extract(month from timestamp '2018-11-12 15:36:01') as month,
       extract(day from timestamp '2018-11-12 15:36:01') as day,  
       extract(minute from timestamp '2018-11-12 15:36:01') as minute,
       extract(second from timestamp '2018-11-12 15:36:01') as second
from dual;

select months_between(to_date('2018-11-12', 'yyyy-mm-dd'),
                      to_date('2017-11-12', 'yyyy-mm-dd')) as zs, --整数
       months_between(to_date('2018-11-12', 'yyyy-mm-dd'),
                      to_date('2017-10-11', 'yyyy-mm-dd')) as xs, --小数
       months_between(to_date('2017-11-12', 'yyyy-mm-dd'),
                      to_date('2018-10-12', 'yyyy-mm-dd')) as fs --负数
from dual;

select sysdate, --当前时间
       round(sysdate, 'yyyy') as year, --按年
       round(sysdate, 'mm') as month, --按月
       round(sysdate, 'dd') as day, --按天
       round(sysdate) as mr_day, --默认不填按天
       round(sysdate, 'hh24') as hour --按小时
from dual;

select sysdate, --当前时间
       trunc(sysdate, 'yyyy') as year, --按年
       trunc(sysdate, 'mm') as month, --按月
       trunc(sysdate, 'dd') as day, --按天
       trunc(sysdate) as mr_day, --默认不填按天
       trunc(sysdate, 'hh24') as hour --按小时
from dual;

数值函数

转换函数

其中数值的格式f可以参考下表：

聚合函数

select classno, avg(t.age) from stuinfo t where t.age < 30 group by t.classno;

select classno, sum(age), count(*), sum(age) / count(*), avg(age) from stuinfo t where t.age < 30 group by t.classno;