Oracle学习笔记—— 常见函数

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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,6,0) from t_user3;
rpad() 向右补全字符串:select rpad(stuno,6,0) 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,23,2,2) 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 Precision8

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

09、,、$、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) yearmonthday 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%

 

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