oracle中CAST函数使用简介

忘是亡心i 2023-06-17 14:59 8阅读 0赞

CAST()函数可以进行数据类型的转换。

CAST()函数的参数有两部分，源值和目标数据类型，中间用AS关键字分隔。

以下例子均通过本人测试。

一、转换列或值

语法：cast( 列名/值 as 数据类型 )

用例：

1）、转换列

\--将empno的类型（number）转换为varchar2类型。

select cast(empno as varchar2(10)) as empno from emp;

EMPNO  
\----------  
7369  
7499  
7521  
...

2)、转换值

\--将字符串转换为整型。  
SELECT CAST('123' AS int) as result from dual;

RESULT  
  ---

123  
返回值是整型值123。

\--如果试图将一个代表小数的字符串转换为整型值，又会出现什么情况呢？  
SELECT CAST('123.4' AS int) as result from dual;

RESULT  
\--------

123

SELECT CAST('123.6' AS int) as result from dual;

RESULT  
\--------

124  
从上面可以看出，CAST()函数能执行四舍五入操作。

\--截断小数

SELECT CAST('123.447654' AS decimal(5,2)) as result from dual;

RESULT  
\-----------  
 123.45  
decimal(5,2)表示值总位数为5，精确到小数点后2位。  
SELECT CAST('123.4' AS decimal) as result from dual;  
结果是一个整数值：  
123  
二、转换一个集合

语法：cast( multiset(查询语句) as 数据类型 )

1)转换成table

例子：

\--学生成绩表

create table stu\_score  
(stu\_no varchar2(50),--学号  
 score  number--总分  
 );  
insert into stu\_score values('201301',67);  
insert into stu\_score values('201302',63);  
insert into stu\_score values('201303',77);  
insert into stu\_score values('201304',68);  
insert into stu\_score values('201305',97);  
insert into stu\_score values('201306',62);  
insert into stu\_score values('201307',87);  
commit;

\------------------------------------------

select \* from stu\_score;

学号         分数

\--------   ----------  
201301       67  
201302       63  
201303       77  
201304       68  
201305       97  
201306       62  
201307       87

\--奖学金表。

\--奖学金表规定了名次，每个名次的人数和奖金。

create table scholarship  
(  
stu\_rank   varchar(10),--名次  
stu\_num     int,--限定人数  
money       number--奖金  
);  
insert into scholarship values('1',1,'1000');  
insert into scholarship values('2',2,'500');  
insert into scholarship values('3',3,'100');  
commit；

\-----------------------------------------------

select \* from scholarship;

名次                                          人数     奖金  
\---------- --------------------------------------- ----------  
1                                              1       1000  
2                                              2        500  
3                                              3        100

现在要根据成绩表的成绩降序排列，按奖学金表的名额确定排名和奖金。排名时不考虑相同成绩。  
排名的结果应该如下：  
学号          成绩        名次   奖金  
201305        97          1        1000  
201307        87           2        500  
201303        77          2         500  
201304        68          3         100  
201301        67          3         100  
201302        63          3         100

SELECT c.stu\_no,c.score,b.stu\_rank,b.money  
  FROM (SELECT c.\*,ROW\_NUMBER() OVER(ORDER BY score DESC) rn FROM stu\_score c) c  
      ,(SELECT b.stu\_rank,b.money,ROW\_NUMBER() OVER(ORDER BY b.stu\_rank) rn  
         FROM scholarship b  
            , TABLE( CAST( MULTISET( SELECT NULL  
                                      FROM DUAL  
                                   CONNECT BY LEVEL <= b.stu\_num  
                                   )  
                            AS SYS.ODCIVARCHAR2LIST )   
                           )  
       ) b  
WHERE c.rn=b.rn;

执行结果如下：

STU\_NO                                                  SCORE      STU\_RANK        MONEY  
\-------------------------------------------------- ----------         ----------          ----------  
201305                                                     97                     1                1000  
201307                                                     87                     2                 500  
201303                                                     77                     2                 500  
201304                                                     68                     3                 100  
201301                                                     67                     3                 100  
201302                                                     63                     3                 100

通过对比发现，确实达到了目的。

此外cast还能转化成collection，varray，此时都需要记过multiset集合函数一起使用。