oracle03

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不同数据类型之间的显式转换
to-char(日期，'日期模式')：把日期型数据转换为字符串
select ename,to_char(hiredate,'yyyy/MM/DD') from emp

ENAME      TO_CHAR(HI
---------- ----------
SMITH      1980/12/17
ALLEN      1981/02/20
WARD       1981/02/22
JONES      1981/04/02
...
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select ename"Name",sal"Salary",
to_char(hiredate,'fmDdspth "of" Month Year fmHH:MI:SS: AM')hiredate from emp

Name           Salary HIREDATE
----------   ------- ----------------------------------------------------
SMITH             800 Seventeenth of 12月 Nineteen Eighty 12:00:00: 上午
ALLEN            1600 Twentieth of 2月 Nineteen Eighty-One 12:00:00: 上午
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to_char(数字，'数字模式')

select ename"姓名",to_char(sal*12,'l999999.00')"工资" from emp

姓名       工资
---------- --------------------
SMITH                 ￥9600.00
ALLEN                ￥19200.00

to_number(字符串,'模式'):

select to_number(65464.1616,'9999999999.00000')from dual

TO_NUMBER(65464.1616,'9999999999.00000')
----------------------------------------
65464.1616

to_date(字符串,'模式'):
SELECT TO_DATE('2006-05-01 19:25:34', 'YYYY-MM-DD HH24:MI:SS') FROM DUAL

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空值: 运算符 is null / is not null

SQL> select empno,ename,sal,job,comm from emp where comm is null;

    EMPNO ENAME             SAL JOB             COMM
---------- ---------- ---------- --------- ----------
7369 SMITH             800 CLERK
7566 JONES            2975 MANAGER

空值排序：默认的order by 升序，NULL值排最后
order by desc 降序，NULL值排最前空值无限大！

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