union与cast

union 2个表的2个不同类型的字段会导致查不出数据，需要转换类型，如果使用to_char或者to_number，也可以使用cast进行类型转换

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.

注意：cast也会默认进行四舍五入

SQL> create table test_1(a varchar(10));
Table created.

SQL> insert into test_1 values ('55.78554');
1 row created.

SQL> select cast(a as int) from test_1;
CAST(AASINT)
------------
55.78554

SQL> select cast(a as number(8,4)) from test_1;
CAST(AASNUMBER(8,4))
--------------------
55.7855

SQL> select cast(a as number(8,3)) from test_1;
CAST(AASNUMBER(8,3))
--------------------
55.786

关于cast与decimal

SQL> SELECT CAST('123.456' as decimal(38, 2))from dual;
 
          CAST('123.456'ASDECIMAL(38,2))
----------------------------------------
                                  123.46

SQL> SELECT CAST('123.456' as decimal) from dual;
 
               CAST('123.456'ASDECIMAL)
---------------------------------------
                                    123