oracle 语法

Select decode（columnname，值1,翻译值1,值2,翻译值2,...值n,翻译值n,缺省值）

From talbename

intersect ，union， union all， minus 
select * from emp intersect select * from emp where deptno=10 ;
select * from emp minus select * from emp where deptno=10;
select * from emp where deptno=10 union select * from emp where deptno in (10,20); --不包括重复行 
select * from emp where deptno=10 union all select * from emp where deptno in (10,20); --包括重复行

1. 日期和字符转换函数用法（to_date,to_char）
         
select to_char(sysdate,‘yyyy-mm-dd hh24:mi:ss‘) as nowTime from dual;   //日期转化为字符串   
select to_char(sysdate,‘yyyy‘) as nowYear   from dual;   //获取时间的年   
select to_char(sysdate,‘mm‘)    as nowMonth from dual;   //获取时间的月   
select to_char(sysdate,‘dd‘)    as nowDay    from dual;   //获取时间的日   
select to_char(sysdate,‘hh24‘) as nowHour   from dual;   //获取时间的时   
select to_char(sysdate,‘mi‘)    as nowMinute from dual;   //获取时间的分   
select to_char(sysdate,‘ss‘)    as nowSecond from dual;   //获取时间的秒

select to_date(‘2004-05-07 13:23:44‘,‘yyyy-mm-dd hh24:mi:ss‘)    from dual//

一年的第几天      
   select TO_CHAR(SYSDATE,‘DDD‘),sysdate from dual

查找月的第一天,最后一天
     SELECT Trunc(Trunc(SYSDATE, ‘MONTH‘) - 1, ‘MONTH‘) First_Day_Last_Month,
       Trunc(SYSDATE, ‘MONTH‘) - 1 / 86400 Last_Day_Last_Month,
       Trunc(SYSDATE, ‘MONTH‘) First_Day_Cur_Month,
       LAST_DAY(Trunc(SYSDATE, ‘MONTH‘)) + 1 - 1 / 86400 Last_Day_Cur_Month
   FROM dual;

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;

四.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;

2, to_date()[将字符类型转换为日期类型]
insert into emp(empno,hiredate) values(8000,to_date(‘2004-10-10‘,‘yyyy-mm-dd‘));

3, to_number() 转换为数字类型
select to_number(to_char(sysdate,‘hh12‘)) from dual; //以数字显示的小时数

1.字符函数

（1）concat(str1,str2)字符串拼接函数

select concat(‘Hello ‘,‘World‘) from dual;
--等价于
select ‘Hello ‘||‘World‘ from dual;

（2）initcap(str)将每个单词首字母大写，其他字母小写

select initcap(‘hello world!‘) from dual; --返回结果为‘Hello World!‘
select initcap(‘HELLO WORLD!‘) from dual; --返回结果为‘Hello World!‘