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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