1. 字符串類型及函數

字符類型分 3 種，char(n) 、varchar(n)、varchar2(n) ；
char(n)固定長度字符串，假如長度不足 n，右邊空格補齊；
varchar(n)可變長度字符串，假如長度不足 n，右邊不會補齊；
varchar2(n)可變長度字符串，Oracle 官方推薦使用，向後兼容性好；

char(n) VS varchar2(n)
char(n)查詢效率相對較高，varchar2(n)存儲空間相對較小；

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

lpad() 向左補全字符串：select lpad(stuno,6,‘0‘) from t_user3;
rpad() 向右補全字符串：select rpad(stuno,6,‘0‘) from t_user3;
lower() 返回字符串小寫：select lower(userName) from t_user3;
upper() 返回字符串大寫：select upper(userName) from t_user3;
initcap() 單詞首字符大寫：select initcap(userName) from t_user3;
length() 返回字符串長度：
 
select length(password) from t_user3;
substr() 截取字符串：select substr(userName,1,2) from t_user3;
instr() 獲取字符串出現的位置：select instr(password,‘23‘,2,2) from t_user3;
ltrim() 刪除左側空格:select ltrim(userName) from t_user3;
rtrim() 刪除右側空格:select rtrim(userName) from t_user3;
trim() 刪除兩側空格:select trim(userName) from
 
 t_user3;
concat() 串聯字符串：select concat(userName,password) from t_user3;
reverse() 反轉字符串：select reverse(userName) from t_user3;

2. 數值類型及函數

number 是 oracle 中的數據類型；number(precision,scale)；
Precision，scale 均可選；
Precision 代表精度，sacle 代表小數位的位數；Precision 範圍[1,38] scale 範圍[-84,127]

舉例： 12345.678 Precision 是 8

scale 是 3；


常用方法：
abs() 求絕對值；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()正負性：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() 格式化數值：常見的字符匹配有

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. 日期類型及函數

Date 和 timestamp（時間戳）
Date 包含信息 century（世紀信息） year 年 month 月 day 日 hour 小時 minute 分鐘 second 秒
Timestamp 一般用於日期時間要求非常精確的情況，精確到毫秒級；
insert into t_date values(1,sysdate,systimestamp);

下面重點講 date 類型的常用函數：
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後一天 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 返回兩個日期的相差月數 select months_between(d1,d3) from t_date where id=1;
next_day 返回特定日期之後的一周內的日期：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 返回日期的某個域：
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 將日期轉換成字符串：
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. 其他常用處理函數

常用的聚合函數：
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 統計記錄數：select count(ename) from emp ;

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

Oracle 中的運算：
select 2+1 from dual;
select 2-1 from dual;
select 2*1 from dual;
select 2/1 from dual;

條件判斷式：
Between and 範圍查詢：
select * from emp where sal between 900 and 1500;
select * from emp where sal>=900 and sal<=1500;

In 集合範圍：
select ename,hiredate from emp where ename in (select distinct ename from bonus)

Like 模糊查詢：
select * from emp where ename like ‘%M%‘
select * from emp where ename like ‘M%‘
select * from emp where ename like ‘_M%‘

（四）Oracle學習筆記—— 常見函數