Oracle的Cast的用法


		Oracle的Cast的用法
 
经常在前辈们的SQL中可以看到CAST的用法,例如:  www.zzzyk.com  
WITH T AS(SELECT LEVEL||' ' L FROM DUAL CONNECT BY LEVEL<=5),
A(A,V) AS(SELECT CAST(L AS VARCHAR(30)),1 FROM T
UNION ALL
SELECT A||L,V+1 FROM A, T WHERE INSTR(A,L)=0 AND V<5
)
SELECT * FROM A;
例如上面的with递归语句，但是CAST因为用的比较少，今天就先给自己扫个盲了，引用Oracle的官方文档中对于CAST的描述如下：
CAST
CAST converts values from one data type to another.
Return Value
The data type specified by type_name.
Syntax
CAST(expr AS type_name)
Arguments
expr can be an expression in one of the data types.
type_name is one of the data types listed in Table1-2，"Dimensional Data Types".
Table 3-1 shows which data types can be cast into which other built-in data types. NUMBER includes NUMBER, DECIMAL, and INTEGER. DATETIME includes DATE,TIMESTAMP, TIMESTAMP WITH TIMEZONE, and TIMESTAMP WITH LOCAL TIMEZONE. INTERVAL includes INTERVAL DAY TO SECOND and INTERVAL YEAR TO MONTH.
Table 3-1 Compatible Data Types
From To BINARY_FLOAT, BINARY_DOUBLE To CHAR, VARCHAR2 To NUMBER To DATETIME, INTERVAL To NCHAR, NVARCHAR2
BINARY_FLOAT, BINARY_DOUBLE
yes
yes
yes
no
yes
CHAR, VARCHAR2
yes
yes
yes
yes
no
NUMBER
yes
yes
yes
no
yes
DATETIME, INTERVAL
no
yes
no
yes
yes
NCHAR, NVARCHAR2
yes
no
yes
no
yes
 
Example
CAST('123.4567' AS NUMBER(10,2)) returns the value 123.46.
通过上面的描述,我们就可以知道CAST可以将一种类型转换为另外一种类型。
比如,将字符串类型转换为NUMBER(10,2)类型，而不仅仅是限于使用用to_number、to_char（）以及to_date（）类型，上述表Table 3-1代表CAST是否能够用于该类型的相互转换。
例子调用：
ChenZw> SELECT CAST('123.4567' AS NUMBER(10,2))  AS NUM FROM DUAL;