（四）Oracle学习笔记—— 常见函数

1. 字符串类型及函数

字符类型分 3 种，char(n) 、varchar(n)、varchar2(n) ；
char(n)固定长度字符串，假如长度不足 n，右边空格补齐；
varchar(n)可变长度字符串，假如长度不足 n，右边不会补齐；
varchar2(n)可变长度字符串，Oracle 官方推荐使用，向后兼容性好；

char(n) VS varchar2(n)
char(n)查询效率相对较高，varchar2(n)存储空间相对较小；

select length(userName) as charlength,length(password) as varchar2length from t_user3;

lpad() 向左补全字符串：select lpad(stuno,6,'') from t_user3;
rpad() 向右补全字符串：select rpad(stuno,6,'') from t_user3;
lower() 返回字符串小写：select lower(userName) from t_user3;
upper() 返回字符串大写：select upper(userName) from t_user3;
initcap() 单词首字符大写：select initcap(userName) from t_user3;
length() 返回字符串长度：select length(password) from t_user3;
substr() 截取字符串：select substr(userName,1,2) from t_user3;
instr() 获取字符串出现的位置：select instr(password,'',2,2) from t_user3;
ltrim() 删除左侧空格:select ltrim(userName) from t_user3;
rtrim() 删除右侧空格:select rtrim(userName) from t_user3;
trim() 删除两侧空格:select trim(userName) from t_user3;
concat() 串联字符串：select concat(userName,password) from t_user3;
reverse() 反转字符串：select reverse(userName) from t_user3;

2. 数值类型及函数

number 是 oracle 中的数据类型；number(precision,scale)；
Precision，scale 均可选；
Precision 代表精度，sacle 代表小数位的位数；Precision 范围[1,38] scale 范围[-84,127]

举例： 12345.678 Precision 是 8

scale 是 3；

常用方法：
abs() 求绝对值；select abs(n1) from t_number where id=1;
round() 四舍五入：select round(n1,2) from t_number where id=1;
ceil() 向上取整：select ceil(n1) from t_number where id=2;
floor 向下取整：select floor(n1) from t_number where id=2;
Mod()取模：select mod(5,3) from dual;
Sign()正负性：select sign(n1) from t_number where id=1;
Sqrt() 求平方根：select sqrt(9) from dual;
Power()求乘方：select power(2,3) from dual;
Trunc()截取：select trunc(123.456,2) from dual;

To_char() 格式化数值：常见的字符匹配有

0、9、，、$、FM、L、C

select to_char(123.45,'0000.000') from dual;
select to_char(123.45,'9999.999') from dual;
select to_char(123123,'99,999,999.99') from dual;
select to_char(123123.3,'FM99,999,999.99') from dual;
select to_char(123123.3,'$99,999,999.99') from dual;
select to_char(123123.3,'L99,999,999.99') from dual;
select to_char(123123.3,'99,999,999.99C') from dual;

3. 日期类型及函数

Date 和 timestamp（时间戳）
Date 包含信息 century（世纪信息） year 年 month 月 day 日 hour 小时 minute 分钟 second 秒
Timestamp 一般用于日期时间要求非常精确的情况，精确到毫秒级；
insert into t_date values(1,sysdate,systimestamp);

下面重点讲 date 类型的常用函数：
select sysdate from dual;
select systimestamp from dual;

Add_months 添加月份 select add_months(d1,2) from t_date where id=1;
Last_day 返回指定日期月份的最后一天 select last_day(d1) from t_date where id=1;

update t_date set d3=to_date('2016-12-20','YYYY-MM-DD') where id=1;
update t_date set d3=to_date('2016-12-20 18:31:34','YYYY-MM-DD HH24:MI:SS') where id=1;

Months_between 返回两个日期的相差月数 select months_between(d1,d3) from t_date where id=1;
next_day 返回特定日期之后的一周内的日期：select next_day(d1,2) from t_date where id=1;
Trunc 截取日期：
select trunc(d1,'YYYY') from t_date where id=1;
select trunc(d1,'MM') from t_date where id=1;
select trunc(d1,'DD') from t_date where id=1;
select trunc(d1,'HH') from t_date where id=1;
select trunc(d1,'MI') from t_date where id=1;

Extract 返回日期的某个域：
select extract(year from sysdate) from dual;
select extract(month from sysdate) from dual;
select extract(day from sysdate) from dual;
select extract(Hour from systimestamp) from dual;
select extract(minute from systimestamp) from dual;
select extract(second from systimestamp) from dual;

To_char 将日期转换成字符串：
select to_char(d1,'YYYY-MM-DD') from t_date where id=1;
select to_char(d1,'YYYY-MM-DD HH24:MI:SS') from t_date where id=1;

4. 其他常用处理函数

常用的聚合函数：
Max 求最大值：select max(sal) from emp;
Min 求最小值：select min(sal) from emp;
Avg 求平均值：select avg(sal) from emp;Sum 求和：select sum(sal) from emp ;
Count 统计记录数：select count(ename) from emp ;

Nvl 空值处理：select ename,nvl(sal,0) from emp;
rownum
Oracle 分页：select * from (select a.*,rownum rn from (select * from emp) A where rownum<=10) where rn>5;

Oracle 中的运算：
select 2+1 from dual;
select 2-1 from dual;
select 2*1 from dual;
select 2/1 from dual;

条件判断式：
Between and 范围查询：
select * from emp where sal between 900 and 1500;
select * from emp where sal>=900 and sal<=1500;

In 集合范围：
select ename,hiredate from emp where ename in (select distinct ename from bonus)

Like 模糊查询：
select * from emp where ename like '%M%'
select * from emp where ename like 'M%'
select * from emp where ename like '_M%'