ORACLE 常用函数

一）日期函数[重点掌握前四个日期函数]

1,add_months[返回日期加(减)指定月份后(前)的日期]
select sysdate S1,add_months(sysdate,10) S2,
add_months(sysdate,5) S3 from dual;
2,last_day [返回该月最后一天的日期]
select last_day(sysdate) from dual;
3,months_between[返回日期之间的月份数]
select sysdate S1, months_between('1-4月-04',sysdate) S2,
months_between('1-4月-04','1-2月-04') S3 from dual
4,next_day(d,day):返回下个星期的日期,day为1-7或星期日-星期六,1表示星期日
select sysdate S1,next_day(sysdate,1) S2,
next_day(sysdate,'星期日') S3 FROM DUAL
5,round[舍入到最接近的日期](day:舍入到最接近的星期日)
select sysdate S1,
round(sysdate) S2 ,
round(sysdate,'year') YEAR,
round(sysdate,'month') MONTH ,
round(sysdate,'day') DAY from dual
6,trunc[截断到最接近的日期]
select sysdate S1,
trunc(sysdate) S2,
trunc(sysdate,'year') YEAR,
trunc(sysdate,'month') MONTH ,
trunc(sysdate,'day') DAY from dual
7,返回日期列表中最晚日期
select greatest('01-1月-04','04-1月-04','10-2月-04') from dual
 
 
二）字符函数(可用于字面字符或数据库列)

1，字符串截取
select substr('abcdef',1,3) from dual
2,查找子串位置
select instr('abcfdgfdhd','fd') from dual
3,字符串连接
select 'HELLO'||'hello world' from dual;
4, 1)去掉字符串中的空格
select ltrim(' abc') s1,
rtrim('zhang ') s2,
trim(' zhang ') s3 from dual
2)去掉前导和后缀
select trim(leading 9 from 9998767999) s1,
trim(trailing 9 from 9998767999) s2,
trim(9 from 9998767999) s3 from dual;
5,返回字符串首字母的Ascii值
select ascii('a') from dual
6,返回ascii值对应的字母
select chr(97) from dual
7,计算字符串长度
select length('abcdef') from dual
8,initcap（首字母变大写）,lower(变小写),upper(变大写)
select lower('ABC') s1,
upper('def') s2,
initcap('efg') s3 from dual;
9,Replace
select replace('abc','b','xy') from dual;
10,translate
select translate('abc','b','xx') from dual; -- x是1位
11,lpad [左添充] rpad [右填充](用于控制输出格式)
select lpad('func',15,'=') s1, rpad('func',15,'-') s2 from dual;
select lpad(dname,14,'=') from dept;
12, decode[实现if ..then逻辑]
select deptno,decode(deptno,10,'1',20,'2',30,'3','其他') from dept;
 
三）数字函数

1，取整函数（ceil向上取整,floor向下取整）
select ceil(66.6) N1,floor(66.6) N2 from dual;
2,取幂(power)和求平方根(sqrt)
select power(3,2) N1,sqrt(9) N2 from dual;
3,求余
select mod(9,5) from dual;
4,返回固定小数位数(round:四舍五入，trunc:直接截断)
select round(66.667,2) N1,trunc(66.667,2) N2 from dual;
5,返回值的符号(正数返回为1,负数为-1)
select sign(-32),sign(293) from dual;