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HomeDatabaseMysql TutorialOracle基础教程:单行函数—类型转换函数

Oracle基础教程:单行函数mdash;类型转换函数

不同数据类型之间的转换
在赋值语句中数据类型会隐式转换,隐式转换应当尽量避免,因为写出来的SQL难理解,随着时间流逝自己都很难理解
特别是嵌入到大型程序中时,另外,Oracle升级会修改隐式转换的规则,这会使程序移植遇到问题.
尤其是索引列不能使用隐式转换 那样就不走索引了

赋值语句中oracle的自动转换规则:
 1.将变长字符型(varchar2)或定长字符型(char)转换成数值型(number)
 2.将变长字符型(varchar2)或定长字符型(char)转换成日期型(date)
 3.将数值型(number)转换成变长字符型(varchar2)
 4.将日期型(date)转换成变长字符型(varchar2)

表达式中oracle的自动转换规则:
 1.将变长字符型(varchar2)或定长字符型(char)转换成数值型(number)
 2.将变长字符型(varchar2)或定长字符型(char)转换成日期型(date)

 

转换成功的条件:
 在将字符型数据转换成数字型时,要保证字符型数据为有效的数.
 在将字符型数据转换成日期型时,要保证字符型数据位有效的日期.
 
SQL> select ename,sal from emp where sal > '4000';

ENAME    SAL
---------- ----------
KING   5000

SQL>   拿字符和数字比较 oracle自动将字符串转成了数字

SQL> select ename,hiredate from emp where hiredate = '17-NOV-81';

ENAME    HIREDATE
---------- ------------------
KING    17-NOV-81

SQL>   拿字符串充当日期 oracle自动将字符串转成了日期

不能直接转换
 数值型和日期型之间不能直接转换,必须将其中之一先转换为字符型,之后再转换为另一种类型

 

显示转换函数:
 to_char
 to_number
 to_date
 

 
to_char(日期,'fmt') 
 将日期型数据转换成字变长字符串.fmt为日期格式
 日期格式必须用单引号扩起来
 常用的时间格式:
  YYYY  数字年
  YEAR  英文年
  MM    数字月
  MONTH 英文月
  DY    缩写的星期
  DAY   完整的星期
  DD    数字日期
  hh    12小时
  HH24  24小时
  MI    分钟
  SS    秒

idle> select ename,to_char(hiredate,'YYYY-MM-DD') from emp where ename='SCOTT';

ENAME    TO_CHAR(HI
---------- ----------
SCOTT    1987-04-19

idle>  将日期型转换成了字符型

to_char(数字,'fmt')
 将数字转换成变长字符串.fmt为数字格式
 9 一个数字  给定的9个数不足 会显示#
 0 显示前导0
 $ 美元符号
 L 本地货币符号
 . 小数点
 , 千位符

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