课堂整理

--绝对值
select abs(-100) from dual;

--取余
select mod(8 ,3) from dual;

--上限值
select ceil (12.5) from dual;

--下限值
select floor (12.5) from dual;

--四舍五入
select round (12.456,1) from dual;

--(截)取整
select trunc (12.456,0) from dual;

--字符串长度
select length (‘asdadad‘) from dual;

select SNAME,length(SNAME) from T_HQ_STUDENT;

select SNAME, substr(SNAME, 1, 2) from T_HQ_STUDENT;  --第一个数字是起始位置，第二个数字是截取长度

--字符串连接
select concat(‘sa‘,‘sd‘) from dual;

select concat(‘sa‘,concat(‘sa‘,‘sd‘)) from dual;

select ‘sa‘||‘sad‘||‘sda‘ from dual;

--查找字符串
select instr (‘asdfgh‘,‘d‘) from dual;

select instr (‘asdfsdfdgdh‘,‘d‘,3) from dual;

--大小写转换
select upper (‘assd‘), lower (‘DFFD‘) from dual;  --upper 小写换大写；lower 大写换小写

select initcap (‘this is a test‘) from dual;

--替换
select replace (‘asdfg‘,‘as‘,‘13‘) from dual;

--填充
select lpad(‘as‘,5,‘d‘) from dual;  --填充左边

select rpad(‘as‘,4,‘d‘) from dual;  --填充右边

--删除
select trim(‘ asdfg  ‘) from dual;    --去空格

select ltrim (‘ asdfg  ‘) from dual;  --去左空格

select rtrim(‘ asdfg  ‘) from dual;   --去右空格

--删除字符

select trim (leading‘a‘ from ‘aasdsd‘) from dual;

select trim (trailing‘a‘ from ‘aasdsda‘) from dual;

select trim (both‘a‘ from ‘aasdsda‘) from dual;

--当前时间
select sysdate from dual;


--月份加减
select add_months(sysdate,2) from dual;

select add_months(sysdate,-2) from dual;



select last_day(sysdate) from dual;


select last_day(sysdate)+ 10 from dual;

--转换函数
select cast (‘123‘as number) + 123 from dual;

select cast(1233 as varchar2(4)) from dual;

select cast(1233 as varchar2(4)) + 123 from dual;

select ‘123‘|| 123 from dual;

select cast(sysdate as varchar2(20)) from dual;

select to_char(sysdate,‘yyyy-mm-dd‘) from dual;

select to_char(sysdate,‘yyyy-mm-dd HH24:mi:ss‘) from dual;

select to_char(123.456, ‘999.99‘) from dual; --不常用

select to_date(‘2015-10-29‘,‘yyyy-mm-dd‘) from dual;