oracle中CAST函數使用簡介

CAST()函數能夠進行數據類型的轉換。

CAST()函數的參數有兩部分，源值和目標數據類型，中間用AS關鍵字分隔。

如下例子均經過本人測試。

1、轉換列或值

語法：cast( 列名/值 as 數據類型 )

用例：

1）、轉換列

--將empno的類型（number）轉換爲varchar2類型。

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

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

2)、轉換值

--將字符串轉換爲整型。
SELECT CAST('123' AS int) as result from dual;

  RESULT
 ---

 123
返回值是整型值123。

--若是試圖將一個表明小數的字符串轉換爲整型值，又會出現什麼狀況呢？
SELECT CAST('123.4' AS int) as result from dual;

 RESULT
--------

 123

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

 RESULT
--------

 124
從上面能夠看出，CAST()函數能執行四捨五入操做。

--截斷小數

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

 RESULT
-----------
 123.45
decimal(5,2)表示值總位數爲5，精確到小數點後2位。
SELECT CAST('123.4' AS decimal) as result from dual;
結果是一個整數值：
123
2、轉換一個集合

語法：cast( multiset(查詢語句) as 數據類型 )

1)轉換成table

例子：

--學生成績表

create table stu_score
(stu_no varchar2(50),--學號
 score number--總分
 );
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;

學號        分數

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

--獎學金錶。

--獎學金錶規定了名次，每一個名次的人數和獎金。

create table scholarship
(
stu_rank  varchar(10),--名次
stu_num    int,--限定人數
money      number--獎金
);
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                                          人數    獎金
---------- --------------------------------------- ----------
1                                              1      1000
2                                              2       500
3                                              3       100

如今要根據成績表的成績降序排列，按獎學金錶的名額肯定排名和獎金。排名時不考慮相同成績。
排名的結果應該以下：
學號        成績      名次  獎金
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;

執行結果以下：

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

經過對比發現，確實達到了目的。

此外cast還能轉化成collection，varray，此時都須要記過multiset集合函數一塊兒使用。