--##字符函数,大小写处理函数
SELECT LOWER('ABcDe') 
FROM dual;  --  abcde，全部转换为小写

SELECT UPPER('ABcDe') 
FROM dual;  --  ABCDE，全部转换为大写

SELECT INITCAP('aBcDe') 
FROM dual; --   Abcde，首字母大写


--##字符函数,字符串处理函数
SELECT SUBSTR('ABcDe',2,2)
FROM dual; --   Bc，截取字符串
SELECT SUBSTR('ABcDe',2)
FROM dual; --   BcDe，截取字符串
SELECT SUBSTR('ABcDe',-3,2)
FROM dual; --   cD，截取字符串

SELECT INSTR('ABcDefgdDh','D',3,2)
FROM dual; --   9，从第三个字符串开始查询第二个字符D的位置
SELECT INSTR('ABcDefgdDh','D')
FROM dual; --   4，从第一个字符串开始查询第一个字符D的位置

SELECT LPAD('ABcDe',10,'D')
FROM dual; --   DDDDDABcDe，左侧不足补字符串D
SELECT LPAD('ABcDe',3,'D')
FROM dual; --   ABc

SELECT RPAD('ABcDe',10,'D')
FROM dual; --   ABcDeDDDDD，右侧不足补字符串D
SELECT RPAD('ABcDe',3,'D')
FROM dual; --   ABc

SELECT REPLACE('ABcDeEe','e',8)
FROM dual;    --ABcD8E8，替换字符

SELECT LENGTH('ABcDe')
FROM dual; --   5，计算字符串长度

SELECT CONCAT('Hello','World') 
FROM dual; --   HelloWorld，连接字符串

SELECT TRIM('H' FROM 'HelloWorld') 
FROM dual;    --elloWorld，去除首字母
SELECT TRIM('d' FROM 'HelloWorld') 
FROM dual;    --HelloWorl，去除尾字母
SELECT TRIM('' FROM 'HelloWorld') 
FROM dual;    --，隐藏字符


--数字函数
SELECT ROUND(245.1234) FROM dual; --245
SELECT ROUND(245.1234,2) FROM dual;  --245.12
SELECT ROUND(245.6234,0) FROM dual;  --246
SELECT ROUND(245.1234,-2) FROM dual;  --200
SELECT ROUND(255.1234,-2) FROM dual;  --300

SELECT ROUND(-245.1234,2) FROM dual;  --  -245.12
SELECT ROUND(-245.6234,0) FROM dual;  --  -246
SELECT ROUND(-245.1234,-2) FROM dual;  -- -200
SELECT ROUND(-255.1234,-2) FROM dual;  -- -300

SELECT TRUNC(245.1234,2) FROM dual;  --   245.12
SELECT TRUNC(245.6234,0) FROM dual;  --   245
SELECT TRUNC(245.1234,-2) FROM dual;  --  200
SELECT TRUNC(255.1234,-2) FROM dual;  --  200

SELECT MOD(1600,300) FROM dual;       --  100
SELECT MOD(1500,300) FROM dual;       --  0 


--日期函数
SELECT SYSDATE 
FROM dual;    --  2012-05-12 12:50:18 ，当前日期

SELECT MONTHS_BETWEEN('15-4月-2012','01-1月-2012') 
FROM dual;    --  3.45161290322581，两个日期之间的月数

SELECT ADD_MONTHS(SYSDATE,2)
FROM dual;    --2012-07-12 14:23:32，添加月份到日期字段中

SELECT NEXT_DAY('15-4月-2012',3)
FROM dual;    --2012-04-17 ，添加天数到日期字段中
SELECT NEXT_DAY(SYSDATE,'星期一') 
FROM dual;    

SELECT LAST_DAY(SYSDATE)
FROM dual;    --2012-05-31 14:25:22，本月份的最后一天

SELECT ROUND(SYSDATE,'month') 
FROM dual;    --2012-05-01，月份四舍五入
SELECT ROUND(SYSDATE) 
FROM dual;    --2012-05-13

SELECT TRUNC(SYSDATE,'month') 
FROM dual;    --月份截取
SELECT TRUNC(SYSDATE,'day') 
FROM dual; 


--转化函数
SELECT '123' + 123 FROM dual;         --246，字符串隐式转化

SELECT TO_CHAR(123456.6543,'L099,999.00') AS "Number Format"
FROM dual;                                --若是常量数字，需在前加0
SELECT TO_CHAR(123456.6543,'$099,999.00') AS "Number Format"
FROM dual;
SELECT TO_CHAR(SYSDATE,'DAY DD-MON-YYYY') AS "Data Format"
FROM dual;                  --星期二 15-5月 -2012

SELECT TO_DATE('12-5-2012','DD-MM-YYYY') 
FROM dual;                  --格式字符串必须有。没有意义？

SELECT TO_NUMBER('5432.1234') 
FROM dual;
SELECT TO_NUMBER('15,155,100.56','999,999,999.00') 
FROM dual;                  --变量字符串与格式字符串的格式必须一致


--常规函数
SELECT NVL(NULL,0) FROM dual;     --    0
SELECT NVL(NULL,'N/A') FROM dual; --    N/A
SELECT NVL(123,0) FROM dual;      --    123
SELECT NVL('学生','N/A') FROM dual; --    学生

SELECT NVL2(NULL,0,1) FROM dual;  --    1
SELECT NVL2(123,0,1) FROM dual;  --    0

SELECT NULLIF(1,1) FROM dual;    --    NULL
SELECT NULLIF(2,1) FROM dual;    --    2
SELECT NULLIF('好','不好') FROM dual;    --    好

SELECT COALESCE(NULL,NULL,'不好','好') FROM dual;     -- 不好，参数必须为同一数据类型

--DECODE示例
DECODE 

SELECT last_n