字串函式:

大小寫轉換函式

將字串全部轉為大寫輸出

select lower('Hello WOrld') from dual;

將字串全部轉為小寫輸出

select upper('Hello WOrld') from dual;

將字串中每個單次的第一個字母轉為大寫輸出其它轉為小寫輸出

處理字串的函式

連線連個字串

select concat('Hello','World') from dual;

計算字串長度

select length('HeloWorld') from dual;

計算一個字串在另一個字串中的位置

select instr('HelloWorld','o') from dual;

計算從右開始第二個“o”所在的位置

select instr('HelloWorld','o',-1,2) from dual; 

截斷字串

select substr('HelloWorld',-5) from dual;

左填充

select lpad(sal,10,'*') from emp;

右填充

select rpad(sal,10,'*') from emp;

函式巢狀：

select lpad(rpad('good',11,'*'),17,'*')from dual;

替換字串

select replace('he love you','he','I') from dual;

截掉字串首尾連續的相同的字元

select trim('H' from 'HelloWorld') from dual;

左截斷

select ltrim('HelloWorld','H') from dual;

右截斷

select rtrim('HelloWorld','d') from dual;

列印字串對應的ascii碼

SQL> select ascii('A') from dual;

列印ascii碼對應的字元

select chr(65) from dual;

數字函式:
select round(45.926,2) from dual;
select round(45.926,0) from dual;
select round(45.926) from dual;
select round(45.926,-1) from dual;


select trunc(45.926,2) from dual;
select trunc(45.926,0) from dual;
select trunc(45.926) from dual;
select trunc(45.926,-1) from dual;


select ceil(3.1415927) from dual; --進位取整
select abs(100),abs(-100) from dual; 
select mod(13,4) from dual; --取餘數
select power(2,3) from dual; --2的3次冪


取數字n的符號,大於0返回1,小於0返回-1,等於0返回0 
select sign(123),sign(-100),sign(0) from dual; 


日期函式:
select round(sysdate+10,'month') from dual;
select round(add_months(sysdate,6),'year') from dual;


select trunc(sysdate+10,'month') from dual;
select trunc(sysdate,'year') from dual;


select months_between('20-FEB-81','17-DEC-80') from dual;
select add_months(sysdate,2) from dual;
select next_day(sysdate,'SAT') from dual;
select last_day(sysdate) from dual;


轉換函式:
select to_char(sysdate,'yyyy-mm-dd') from dual;
select to_char(sysdate,'fmyyyy-mm-dd') from dual;
select to_char(sysdate,'year-month-day') from dual;
select to_char(sysdate,'year-mon-dy') from dual;
select to_char(sysdate,'yyyy-mm-dd hh24:mi:ss') from dual;
select to_char(sysdate,'dd "of" mm') from dual;

select to_char(10,'xx') from dual; --轉換為16進位制

select to_char(sal,'l00,000.00') sal from emp;
select to_char(sal,'l99,999.99') sal from emp;
select to_char(sal,'l999.99') sal from emp;

select to_number('$950.00','l999.99') from dual;
select to_number('9c','xx') from dual; --轉換為10進位制
select to_date('2000-05-01','yyyy-mm-dd') from dual;

函式巢狀，列印yy年和rr年
select
to_char(sysdate,'yyyy') curr_year,
to_char(to_date('05','yy'),'yyyy') yy05,
to_char(to_date('95','yy'),'yyyy') yy95,
to_char(to_date('05','rr'),'yyyy') rr05,
to_char(to_date('95','rr'),'yyyy') rr95
from dual;

通用函式:
nvl : expr1空則expr2，expr1不空則expr1;
＊ nvl函式兩個形式引數的資料型別要一致

select ename,sal,comm,sal+nvl(comm,0) from emp;
select ename,nvl(to_char(mgr),'No manager') from emp;

nlv2 : expr1空則expr3，expr1不空則expr2

select ename,nvl2(comm,sal+comm,sal) from emp;

nullif : expr1＝expr2返回空，expr1!＝expr2返回expr1

coalesce(exp1,exp2,exp3,.........) : 從左向右做列舉判斷返回第一個非空表示式的值

條件表示式:
SELECT ename, job, sal,
       CASE job WHEN 'CLERK'  THEN  1.10*sal
                WHEN 'SALESMAN' THEN  1.15*sal
                WHEN 'ANALYST'   THEN  1.20*sal
       ELSE sal END     "REVISED_SALARY"
FROM   emp;

select ename, job, sal,
       decode (job,
                   'CLERK',1.10*sal,
                   'SALESMAN',1.15*sal,
                   'ANALYST',1.20*sal,
               sal) "REVISED_SALARY"
from emp;

系統函式:
檢視當前使用者:
select user from dual;
select SYS_CONTEXT('USERENV','SESSION_USER') from dual;
檢視系統時間:
select sysdate from dual;
檢視客戶端IP地址:
SELECT sys_context('USERENV','IP_ADDRESS')  FROM DUAL;
檢視主機名:
SELECT sys_context('USERENV','HOST')  FROM DUAL;
檢視當前方案:
SELECT sys_context('USERENV','CURRENT_SCHEMA')  FROM DUAL;
檢視當前的安全稽核機制:
SELECT sys_context('USERENV','AUTHENTICATION_TYPE')  FROM DUAL;