Cast函数、round、trunc、ceil

cast 是进行类型转换的, 可以针对各种Oracle数据类型. 修改的是用户的数据类型
例：
SQL> create table t1(a varchar(10));

Table created.

SQL> insert into t1 values ('12.3456');

1 row created.

SQL> select round(a) from t1;

  ROUND(A)
----------
        12

SQL> select round(a,3) from t1;

ROUND(A,3)
----------
    12.346

SQL> select cast(a as int) from t1;

CAST(AASINT)
------------
          12

SQL> select cast(a as number(8,4)) from t1;

CAST(AASNUMBER(8,4))
--------------------
             12.3456


转换为“decimal”类型，如下：
select Cast(1.888888 as decimal) from dual;
----------------------------------
|   Cast(1.888888 as decimal)    |
----------------------------------
|           2                    |
----------------------------------

select cast(1.88888 as decimal(12,2)) from dual;
----------------------------------
|   Cast(1.88888 as decimal)     |
----------------------------------
|           1.89                 |
----------------------------------

========================================================
round只是修改的数据显示格式. 对数据做四舍五入. 类似的函数还有ceil(取此数据的最小整数). trunc[color=red][/color]( 取整函数.)
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代码:
SQL> select round(12.45) round,trunc(12.45) trunc,ceil(12.45) ceil
  2  from dual
  3  /

     ROUND      TRUNC       CEIL
---------- ---------- ----------
        12         12         13

SQL> select round(12.54) round,trunc(12.54) trnc,ceil(12.54) ceil
  2  from dual
  3  /

     ROUND       TRNC       CEIL
---------- ---------- ----------
        13         12         13

SQL>